Soviet director Leonid Gaidai often used this. One of the most famous examples is a scene from his comedy The Diamond Arm where a smuggler who got out of the sea notices that his accomplice got stuck on a tiny island several hundred meters away from the shore. The first smuggler, played by the veteran actor Anatoly Papanov, looks out into the sea, growls "Idiot!" and angrily spits out. In reality, the "Idiot" remark was addressed at the cameraman because he had filmed the scene incorrectly and because of this Papanov had to go back into the cold water. Gaidai liked the genuine anger of that remark, so it was inserted into the final cut.
Norman Mailer directed a small number of films where he threw in unplanned bits:
In Wild 90, an improvisational movie, about twenty-five minutes of the soundtrack became muffled due to a technical error. Mailer decided to release the movie with the soundtrack muffled, rather than redub it, saying it "sounds like everybody is talking through a jockstrap."
During the filming of Maidstone, a movie about a director attempting to become President, Rip Torn attempted to hit Mailer over the head with a hammer. The two of them then fought viciously, all while the cameras rolled. The fight appeared in the movie.
While making Tough Guys Don't Dance, Ryan O'Neal gave a poor line reading which Mailer put into the movie, over the protests of various people, including O'Neal himself, because he felt the poor reading added something to the picture. See it for yourself here.
Groucho Marx ad-libbed frequently; many Marx Brothers movies have noticeable blips where the makers shaved off a few seconds to make room for things like the Animal Crackers speech which begins: "Pardon me for a second while I have a strange interlude."
Some scripts simply had "Harpo Does Something Funny" because his improvisations were often better than what the writers could come up with.
Their first major film The Cocoanuts had to be shot with multiple cameras because every take they did was different, so normal single camera techniques didn't work. The "viaduct" gag was not in the original script of the play the film was based on.
In Animal Crackers the actor playing Mr. Chandler accidentally called Groucho by his own character's name. Both of them were able to improvise off it well enough that the take ended up in the film.
Peter Sellers: according to IMDB, Inspector Clouseau's "rit of fealous jage" line in A Shot in the Dark was an actual slip of the tongue by Sellers. It was so Clouseau-esque, however, that Blake Edwards kept it in.
Actor Robert Prosky described his approach for the restaurant scene in Mrs. Doubtfire as "hold on for dear life" since he never knew exactly what was going to come out of Williams' mouth during any given take. If you watch that scene carefully, you can see Pierce Brosnan struggling not to crack up at Robin's antics, and this is made all the funnier by the fact that Brosnan's character is supposed to be annoyed/angry throughout most of that scene.
Much of the monologue in Good Will Hunting where Robin Williams' character is counseling Matt Damon on relationships was ad-libbed. This is particularly true in a bit where Williams is describing his dead wife and her tendency to be flatulent when sleeping, which is why Will responds by laughing almost hysterically — Matt Damon himself had no idea what was coming. You can also see the camera shaking very slightly, and it's been reported that the cameraman too was laughing. His last words ("Son of a bitch, he stole my line") were also improvised. So was "Fuck you!" "You're the shepherd."
During filming of The Birdcage, Robin Williams and Nathan Lane were so thoroughly into ad-libbing and bouncing off one another that they were forced to promise they'd do one take exactly as scripted before they were allowed, in subsequent takes, to say whatever they wanted. Also, the scene where Robin Williams trips carrying the pot of soup was not supposed to happen, but how hilariously appropriate it was to the mood made it into the film. If you pay enough attention, Hank Azaria nearly loses it at Robin falling.
Genie, the breakout character of Aladdin was originally supposed to be very somber and mellow; he'd appear grant the wishes, not take up too much time. Then they got Robin into the booth. Supposedly there are over four hours of him just going off, with a good third of that unusable because "you can't say that, we're Disney". It was so funny, however, that the character (and script) was rewritten, and the animators told to show whatever impression or visual gag Robin would do in film.
In 28 Days, Gerhardt's speech about forks in the road, salad forks, crab forks and ladles was entirely ad-libbed by Alan Tudyk.
In 300, the memetic delivery of the line "THIS! IS! SPARTA!!!" was actually not that highly emphasized in the comic book novel. Gerard Butler himself made the emphasis when that scene was filmed, and the rest as they say is history.
The famous classical music from 2001: A Space Odyssey was just supposed to be a placeholder used while they edited the movie. But Stanley Kubrick liked it so much that he kept it in as the movie's score.
Alien: Resurrection has a marginal example: one of the scenes in the movie takes place on a basketball court where Ripley was playing a game, and one of the ideas that they had was that Ripley would shoot a basket with her back to the net. Sigourney Weaver practiced that shot quite a lot with her coach — but never from as far away as she would have to on set, and in all the rehearsals, she missed the shot. So when they finally filmed it, it's with the understanding that all she needed to do is put the ball up in the air, and they'll CG it into the net later.
Needless to say, when the ball caught nothing but net, they had to cut the footage right after the shot so as not to include the cries of "holy shit!" that followed.
Two flubbed lines by Robert Redford made it into the final cut of All the President's Men, thanks to Redford's ability to work the mistake into his performance. One was Redford on the phone with a person who spoke only Spanish, asking the others in the newsroom: "Does anybody here speak English?" when he was supposed to ask for someone who spoke Spanish. The other is at the end of a six minute take with Redford on the phone (again). He calls the person he's talking to by the wrong name but keeps going.
In Almost Famous, Penny asks William if he wants to go with her to Morocco, and he answers, "Yes. Wait, ask me again." Penny asks him again, and he responds with an even more enthusiastic "Yes!" This was not in the script; Patrick Fugit, who played William, was simply asking Kate Hudson to do another take of that line, and Cameron Crowe left both takes in.
In American Graffiti the opening where Terry (Charlie Martin Smith) crashes his moped into a garbage can was an accident but left in.
Jim asks Michelle, aka Alyson Hannigan, to the prom. After she talks about using her flute to make her sing and wanting to have sex with Jim, she climbs on top of him and says, "What's my name? Say my name bitch!" This was yet another "tried it differently on the last take" that they threw in. (You can see all the takes on the DVD special features.)
The second film makes a call back to this as Michelle refers to Jim as "My Bitch".
Eugene Levy is known for doing this. Adam Hertz, the screenwriter for the American Pie movies, said when writing for Mr. Levinstein he likes to sketch out what he wants Levy to say and not write too much actual dialogue.
Annie Hall: the scene where Alvy sneezes, blowing away a boxful of his friend's $2000/oz cocaine ran much longer, but was cut back because the laughter from the audience made the rest of the dialog inaudible. The sneeze was real, and unrehearsed.
A few John Belushi moments in Animal House came about like this, particularly in the cafeteria scene. His trip through the buffet line was between takes, but when the crew saw they were told to keep rolling. Moments later, he improvised the "I'm a zit" gag, and the looks of surprise and disgust on the actors are genuine.
In the Armageddon, Bruce Willis improvised the famous line: "The President of The United States just asked us to save the world... anyone want to say 'no?'" Michael Bay liked it so much he made sure they put it in the trailer.
In Attack of the Killer Tomatoes!, the helicopter that they'd rented for the day crashed, and they caught it on film...so they added a line a about a tomato leaping at it.
The actors just got out of the wreck, dusted themselves, and went right on with the scene.
"Well, what do you think?"
"I don't think it will ever fly again."
In Atonement, director Joe Wright reveals in his commentary that the scene just before Robbie discovers the school girls massacre, at the point where he removes his helmet, the weather is cloudy. As he looks up the sky, the sunlight surprisingly shines and gets cloudy again the moment he put his head down.
During the fountain scene, at one point Austin's, ahem, "stream" starts giving out intermittent splashes like a sprinkler. According to the DVD commentary, this was actually a result of the water cannon malfunctioning, but the directors found it so funny they left it in.
Similarly, Scott Evil's little dance at the very end after gloating that he'll get Austin Powers in the same film was not in the original script. Seth Green, the actor for Scott Evil, was just fooling around with the set without realizing that the camera was actually rolling, but the directors found this to be hilarious so they left it in.
The scene with Thorton Melon's secretary taking notes for him in Back to School was supposed to show his son Jason sitting next to her looking disgusted, but the actor simply couldn't stop laughing at Edie McClurg's performance. They decided to leave it in since it works just as well that he is supposed to be laughing in frustrated disbelief instead.
In Back To The Future Part II, the stunt double of Griff's female gang member was injured when she hit the column of the courthouse. They actually used that take in the movie.
In the final shot of Barton Fink, the seagull diving into the water was unplanned.
In Basic Instinct, actors Michael Douglas and Jeanne Tripplehorn were merely rehearsing the sex scene between their characters and didn't even know they were being filmed, but the director liked the footage so much that he put it into the film.
In Tim Burton's Batman, when exploring Wayne Manor with Vicki, Knox (Robert Wuhl) ad-libbed the jokes about the decorative suits of armor Wayne has.
In Being John Malkovich, in the scene where John Malkovich is walking away from John Cusack, a passenger in a car driving by yells "Hey, Malkovich, think fast!" and throws a beer can at Malkovich's head and hits him. Apparently, it was an extra that had gotten drunk and just decided to do it at the spur of the moment. It was kept in, and the extra got a pay raise because he now had a line of dialogue in the film, and qualified him for a SAG card.
During the filming of the chariot race in Ben Hur, Charleton Heston's stunt double Joe Canutt almost flew out of the chariot when it jumped over a wrecked chariot, which was unintentional. The shot was left in with director William Wyler shooting a closeup of Heston climbing back into the cart. Reputedly, the crowd flooding the arena at the end of the chariot race was an unplanned move by enthusiastic extras.
Several scenes in The Film of the Series of Bewitched were directly scripted from development-period improvisations between Nicole Kidman and Will Ferrell, including, in an amusing recursion, the scene where Isabel and Jack "improvise" an interview with a witch. According to Nora Ephron's DVD Commentary, this scene was essentially unchanged from the original improv, right down to Kidman's line, "Do we have to keep doing this?"
In the Blaxploitation ParodyBlack Dynamite, there is a scene where some men in black suits are shooting at Black Dynamite and a man in a large jelly doughnut costume from the car. As they pull up in the car and begin shooting, the car begins rolling away due to the fact that the actor forgot to enable the parking brake. He got it eventually, but the shot was kept in due to the fact that it fit in with the rest of the movie's intentional "Throw it In"s.
According to the DVD info for Black Knight, one of the female lead's faceplants was entirely unintentional, but kept in because it looked awesome.
Roy Batty's "tears in rain" speech from the ending of Blade Runner was actually a mostly-improvised performance by Rutger Hauer that was kept in the final product. According to Hauer and screenwriter David Webb Peoples, the script called for Batty to deliver a two-page speech that explained all of his past adventures in greater detail. After a long night shooting, and with the sun coming up on the final day of filming, Hauer (who had been trying to figure out how he could condense it down) stripped the speech down to its barest minimum and delivered it in one take.
Blazing Saddles: When Jim the gunfighter is telling Bart why he'll never be accepted in Rock Ridge, Gene Wilder's line goes, "These are simple folk, people of the land. The common clay of the new West. You know: morons." Cleavon Little started breaking up, because Wilder had improvised the "morons" part.
Borat: the naked wrestling scene. Sacha Baron Cohen told the director that if he ran short on oxygen from having a 300 lb man sit on his chest he'd hit the mattress three times fast. If you look you'll find he does that about halfway through the fight.
In the classic Bringing Up Baby, Cary Grant's character Dr. David Huxley has lost his clothes and is forced to find whatever he can around Susan's aunt's house to wear home. Naturally, all that is available are a ridiculous pair of hunting boots and a woman's frilly nightgown. When Susan's aunt sees him, she angrily asks him why he's wearing those clothes; impatiently, Grant jumps in the air and shouts "Because I just went GAY all of the sudden!" Whether this was a reference to homosexuality or not is unclear, but it wasn't scripted in any case.
It's 90% likely an intentional reference (also the first use of the term 'gay' to mean homosexual in a Hollywood movie): Note Huxley's next line is that he is "in the middle of 42nd street waiting for a bus"; at the time 42nd & Broadway was New York's seediest area, with lots of cruising homosexuals. (Who, when questioned about their activities by police, would often proffer "I'm just waiting for a bus" as an excuse.)
Bill Murray ad libbed the scene in Caddyshack where he was pretending to win the Masters.
Caine was supposed to be about smuggling. When a stuntman was killed by a shark, it was retitled Shark and recut as an animal attack movie.
The word "Shpadoinkle" was originally a placeholder for the song "It's a Shpadoinkle Day," but when Trey Parker first played the song for friends they loved the word so much that it was kept.
There was a scene where the town's drunken sheriff informs Packer that he's going to be hung at sunrise, then adds "You know what they say about sunrise?", awkwardly pauses for a moment, and just wanders off. According to the commentary, the actor (who actually was drunk) had just forgotten his line and walked off camera — Trey Parker opted to leave it in because he decided that it was funnier than the actual punchline.
The most famous example of all time, in Casablanca:
Rick: Here's lookin' at you, kid.
There's also scene where several of Rick's French customers sing La Marseillaise to drown out the Nazis and break down crying. Most of these extras were Europeans displaced by the Nazis and the crying was genuine.
In the film version of The Cat in the Hat, Mike Meyers is standing in the hallway amidst the house falling down, and one particular beam falls, and Mike jumps and starts looking around, because no one told him it was going to happen.
In Eric Liddell's first scene at the running meeting in Chariots of Fire, Ian Charleson (Liddell) was giving a speech - "I am, and will be whilst I breathe..." - when he was suddenly interrupted by a cow mooing offscreen. Instead of stopping, he smiled and finished his line: "...a Scot." The director liked it and left it in.
In Citizen Kane, Joseph Cotten mispronounced the word "criticism" and quickly corrected himself, due to his exhaustion in acting in the ambitious film at the same time that he was starring in The Philadelphia Story on Broadway. Since his character was drunk in the scene Wells decided to use that take, and in fact you can see that he is initially surprised and then pleased by the mistake. There is a story that Welles intentionally had Cotten kept awake for a very long time, because extreme fatigue resembles drunkenness.
Malcolm McDowell claims Alex's use of the song "Singin' in the Rain" during the rape scene in A Clockwork Orange was an improvisation on his part which Kubrick approved. During rehearsal, the scene had not been working as scripted, so Kubrick told McDowell to try dancing. While trying this, McDowell spontaneously began singing the song. Kubrick realized it worked and immediately left the set to call New York and secure the rights to the song.
In a scene from Clueless, Cher is giving a speech about granting asylum to Haitians. Cher pronounces it "Hate-ians" instead of "Hay-shens." This wasn't scripted - Alicia Silverstone didn't actually know how to pronounce Haitians. The director liked it so much that she told the crew not to correct Silverstone.
At the beginning of the 1965 French film Le Corniaud (The Sucker), the Citroën 2CV driven by Bourvil's character is hit by Louis de Funès's Rolls Royce, which causes it to fall into pieces a la Blues Brothers. Both drivers then proceed to have an argument over the wreckage, during which Bourvil threw the ad-libbed line "Maintenant, elle va marcher beaucoup moins bien, forcément!" ("Now, it will run a lot less well, naturally!"). De Funès had to lower his head to hide his snicker at the unexpected reply in order to not ruin the shot. (Which they couldn't afford, as they had only one self-destructing car available...)
Reportedly, the scene outside the hospital where Joker expresses displeasure at the lack of explosions was unscripted; a majority of the explosives failed to detonate, so Ledger's 'that's it?' shrug and fiddling with the trigger mechanism are all improvised. His running like hell when the rest of the explosives went were also improvised.
The Dark Knight Rises: Tom Hardy improvised the line, "That's a lovely lovely voice," which Bane says while listening to the boy singing the national anthem at the football stadium.
Many, many, many, scenes of Date Night are all improvised, and ad-libbed.
In the 2004 Dawn of the Dead (2004) film, an early scene filmed with a fixed camera on a car roof as it drives around the wrecked apocalyptic neighborhood almost caused an accident when the car drove in front of another vehicle, forcing the other driver to slam their brakes and barely avoid hitting it. Everyone agreed that it added a great touch to the apocalyptic feel of the sequence.
According to the director, much of the humor in Death at a Funeral was based on deliberately exploiting this trope. He explained that scenes would often be repeated until something funny went wrong, and then that take was used.
One of the crashes staged for Death Race unexpectedly sent the stunt car much higher than intended, so it hung up on a billboard rather than smashing into the wall below. The resulting footage looked so badass that the filmmakers went back and equipped one of the other racers' vehicles with a rocket launcher suitable for blowing rival drivers sky-high, purely so they could justify using the clip.
In the 2006 movie Déjà Vu, there's a scene where the Timey Wimey Machine is ramping up to full power. As they're trying to get it to work, the lead machine wonk played by Adam Goldberg yells at his colleagues "I need more cowbell!" Reportedly this was an ad lib by Goldberg that suited the scene so well they left it in, and somewhat amusingly, several reviews singled it out as one of the most entertaining moments of the film.
The famous "squeal like a pig" quote from Deliverance was not present in the source novel or the original script. It was an ad lib cooked up by the actors that the director liked since it didn't contain any of the foul language of the original line, meaning the line wouldn't have to be cut or edited for TV.
Alan Rickman plays Big Bad terrorist Hans Gruber. When he can't get information from a character, he shoots him in cold blood without a second thought. Later, he tells the rest of the terrified hostages, "I wanted this to be professional, efficient, adult, cooperative, not a lot to ask; sadly, your Mr. Takagi couldn't go along, so he won't be joining us for the rest of his life." This line was an ad-lib by Rickman.
He also ad-libbed the idea of eating some of the food from the party buffet while saying the line.
While not as spur-of-the-moment as many examples, the scene where Gruber pretends to be a hostage was written after the filmmakers discovered that Rickman could do an excellent American accent. The filmmakers had been looking for a way to have McClane and Gruber meet face-to-face before the movie's climax, and Rickman's accent provided a way to do that.
In one scene McClane tries to jump between air ducts in an elevator shaft. He misses his mark and just barely clings to one of the lower ducts - which was really an accident by the stuntman, but included in the final cut because it looked authentic.
French canadian comedy Ding et Dong: Le Film has the two dim as lightbulbs protagonists hired as stuntmen for a driving sequence in a movie being filmed. They barely understand enough english to write down the english speaking director's directions, and turn the wrong way right at the start, leaving the stunt course and driving like maniacs in real traffic. When they're finally stopped, the character in the passenger seat looks dizzy and asks "Pis, c'etais-tu correct?" ("So, was that good?") before being arrested by the police. The actor who played the role stated that he had a completely different line to say, but the stunt drive he had just gone through (the driver was a real stuntman) had left him so shaken that he blurted out that line to the real movie director, forgetting that the cameras were still rolling. They loved that take so much that it was used in the final movie. See it here.
In the 1967 film, The Dirty Dozen. Lee Marvin's "Oh, they played an active part alright." line was completely unplanned, as was Ernest Borgnine's reaction of spitting his drink on the floor and coughing.
District 9: Many, if not all of Wikus' lines are improvised. When you consider how beautifully Sharlto Copely acts his part, this becomes really impressive food for thought.
In Django Unchained, Candie (Leonardo DiCaprio) cutting himself before smearing blood on Broomhilda's face was apparently ad-libbed. DiCaprio actually injured his hand while smacking it on the table (and the glass). This is also why nobody reacts to Candie's hand dripping blood, but are all giving each other "are we supposed to pretend we don't notice that?" side-eyes.
General Turgidson's tumble in the War Room was unscripted and accidental.
George C. Scott didn't intend to play Turgidson quite so whacky all of the time. In some of the scenes, after the "official" take, Kubrick would tell him to do it really, really over-the-top to amuse the other castmembers, then stuck that take into the final cut. Scott was initially upset about this until he actually saw the result.
A great deal of Peter Sellers' performance is said to have been improvised, including prominent examples such as President Muffley's "Just as sorry as you are" phone conversation with Premiere Kissov, and the title character's uncontrollable hand. Highly uncharacteristically for Kubrick, who was known to be an extreme perfectionist, who in some cases demand would several hundred takes of a scene to get it just right, wholeheartedly embraced Sellers' improvisations and made sure to keep several cameras on him at all times during filming.
A lot of dialogue in Dog Day Afternoon including Al Pacino yelling "Attica! Attica!" and John Cazale's response when Pacino asks him what country he wants to go after the robbery: "Wyoming."
Supposedly, the scene where Lt. Emily Lake (played by Lauren Holly, as a Naval experiment for having women on submarines) confronts her commander (played by Kelsey Grammer) regarding a sub maneuver that he'd pulled to help her regain her confidence. When leaving, the part where the actress slammed her elbow into the doorjamb, and gave a short hysterical laugh before darting out of the set was not in the script, but kept anyway for extra laughs.
Also supposedly, many of Nitro's lines were ad-libbed. The character, portrayed by Toby Huss, was only supposed to have a couple lines in the beginning of the film.
In reality, he had been in the USA for eight years by this point, and spoke the heavily accented but serviceable English he would for the rest of his life. However, it is entirely possible that two years earlier, when he took the role on Broadway, he was directed syllable-for-syllable and kept the strange results.
The scene in The Dreamers where Isabelle's hair catches fire happened unplanned. Eva Green was supposed to lean forward and kiss Matthew goodnight but accidentally caught her hair on fire on the candle on the table. She didn't let it worry her and acted so natural that Bernardo Bertolucci decided to leave it in as he felt it demonstrated perfectly the cross over in the film that things are about to get a bit crazy!
The "Most Annoying Sound" scene in Dumb and Dumber was unscripted (you can tell because it's clear Jeff Daniels is about to crack up).
In the film version of East of Eden, Cal was supposed to deck Adam after he rejects his present. When shooting, James Dean had the impulse to instead hug Raymond Massey. This became a moment of Enforced Method Acting for Massey who, unpracticed in improvisation, came across exactly as stiff and uncomfortable as Adam ought to be under the circumstances.
Another continuity reference happens at the end of El Dorado when John Wayne snaps at Robert Mitchum for alternating which arm he put his crutch under. Before becoming a big star, Wayne did part-time continuity work in college.
In Enter the Dragon in the scene where O'Hara (Robert Wall) is beaten by Lee, first Lee does a flying kick into Wall... that's right, the actor. Then O'Hara breaks a bottle in an attempt to get back at Lee. What happened on set is that during the bottle scene Wall accidentally cut Lee during one take. Lee having a fairly legendary temper retaliated in a take of the kick scene and that flying kick that "defeats" O'Hara before he breaks the bottles is a real Bruce Lee kick to Wall.
That's been pretty thoroughly jossed as a rumour started by the director Robert Clouse. The cut was an accident caused by Lee slightly misaiming a kick that was supposed to knock the arm out of the way and catching his hand on the broken bottle on the follow through from the kick. The realism of the kick scene was a mutual decision between Wall and Lee to make the fight look more authentic - Wall was a guy who knew how to take a hit and was happy to take a few bruises to make the scene work better.
An In-Universe example occurred in the Even Stevens movie: In the film's beginning, during Ren's valedictorian speech, Louis and Beans (after tricking Donny into letting them go backstage) have a beach ball-shaped device bounce around the podium, with Tugnut (who was asleep during the speech) being ordered by Weskler into grabbing the ball and restoring order. Louis intended for the ball, when exploding, to release confetti. However, Beans (who presumably was the one who actually created the ball) misheard his instructions and said that he thought Louis said "spaghetti", right after the ball detonates and sprays the coach with the brunt of spaghetti and meatballs. Louis then said to Beans that his mishearing things made the prank even better.
Ferris Bueller's Day Off has actress Edie McClurg's famous line, "They think he's a 'righteous dude.'" This was ad-libbed.
In Field of Dreams, Shoeless Joe Jackson accidentally hits a ball close enough to make Ray Kinsella jump aside with a "Whoa!" It also knocks over the nearby bag of baseballs.
When The Narrator exits from the Tyler Durden-controlled police station, he threatens the police officers with a "lead salad." The line was improvised by Edward Norton on the spot, as were the dubious expressions of the threatened officers.
According to the DVD commentary, during the first fight between the Narrator and Durden, where Durden is taunting him to "hit me, hit me in the face" and gets punched in the ear instead, Norton was supposed to take a swing at Brad Pitt's shoulder, but improvised and actually hit Pitt in the ear, resulting in the "Ah, God! Fuck! Why the ear, man?!" line.
The scene with Pitt and Norton hitting golf balls into the trainyard had nothing to do with the original script. The two of them were drunkenly aiming golf balls to hit the catering trucks; Fincher decided to film it and put it in.
In First Blood, protagonist John Rambo jumps off a cliff into a tree, then falls down, hitting branches on the way down, to hit the ground with a blood-curdling scream. That's because Stallone broke three ribs doing the stunt.
The "You know how I know you're gay?" scene sprang from an improvisation about a completely different subject.
The waxing scene: they didn't tell Steve Carell on the first rip that they were actually going to go through with it. Hence his expression, followed by some decidedly out-of-character swearing at the actor who just ripped half his chest hair off. Those wincing looks and glances off-camera from his "buddies" are real. Carell also ad-libbed all the lines he yelled after each rip including "Kelly Clarkson!!" The script for this scene actually read: "Scream, swear, apologize," if memory serves.
In The French Connection, while tailing Sal, the actor playing Sal got too far ahead of the chase car, which was then caught in a traffic jam coming off the Brooklyn Bridge. There's also the car, a '67 Torino, Popeye crashes into. It was actually just some guy on his way to work, driving on a street that was missed when closing traffic. The production crew paid for the repairs to his car.
When Richard Kimble pleads with Deputy Marshall Gerard, "I didn't kill my wife!", Jones ad-libbed his blunt response of "I don't care!"—which promptly became the film's most memorable line.
When Gerard and an extra are hanging around:
Gerard: Newman, what are you doing? Newman: I'm thinking. Gerard: Well, think me up a cup of coffee and a chocolate doughnut with some of those little sprinkles on top — while you're thinking.
Harrison Ford injured his knee during filming, but postponed surgery until filming was complete, feeling (correctly) that the resulting limp would heighten the tension of the chase scenes and emphasize Kimble's vulnerability.
Similar to his actions while filming A New Hope, he also refused to memorize the script for the scene where he's interrogated by the police, allowing his responses to be completely realistic.
R. Lee Ermey's Gunnery Sergeant Hartman, in Full Metal Jacket, pulled the "reach-around" line, when insulting a recruit from Texas, out of his head. Kubrick stopped the filming to ask Ermey what that meant. After it was explained, Kubrick simply said to go with it. Other Hartmann lines were also ad-libbed, with Ermey being one of the relative few that Kubrick, notorious in some circles as a control freak, allowed to go off-script. It helped that Ermey really was a Drill Instructor during Vietnam.
Ermey's entire character in the movie was thrown in, sort of. Ermey sat down with Kubrick as a "technical adviser", and for hours straight he literally just had a stream-of-consciousness moment with every horrible thing he could think of when yelling at new recruits, for later incorporation into the script. Kubrick was so impressed with it all, he later decided to just cut out the middleman and cast him as Hartman. In a way, any time he ad-libs in the movie it's the same as an "executive rewrite."
Ermey actually requested to be cast as Hartmann. Kubrick refused at first, but then Ermey brought in an audition video that consisted of him yelling insults at a line of SAS members while being pelted with oranges from offscreen, not once messing up or hesitating. It convinced Kubrick on the spot.
In Get Smart, the scene after the parachute jump in which 99 grills Max about what he would do if someone pointed a gun at him was taken almost verbatim from dialogue Anne Hathaway ad-libbed for her screen test. The director like it so much, he added it to the film. (From the making-of featurette on the DVD.)
In Ghostbusters, the appearance of Gozer was a last-minute decision by Ivan Reitman, made and announced to the cast right before the scene was filmed. Gozer's appearance in the script is that of Ivo Shandor, the occultist who began the summoning in the 1920s, but Reitman spontaneously came up with the idea of Gozer as an androgynous, otherworldly female, and her costume, equally improvised at the last second, is literally taped-up bubble wrap. The cast thought it was insane and would spoil the finale, and continue to express amazement today at how well it worked. Then again, Paul Reubenswas intended to play the part of Ivo Shandor...
Bill Murray reportedly adlibbed at least some of his lines. The degree varies between different accounts from practically everything he said to just a couple of lines.
The commentary notes that practically every scene had an ad-lib, not just by Bill Murray either. Rick Moranis also ad-libbed much of his dialogue, especially in the party scene, though he worked with the screenwriters to get a vague outline of what was needed.
During the scene where Dana watches the Ghostbusters' cheesy no-budget commercial, watch how Egon steps forward, delivers his line, and awkwardly steps backward while glancing down to make sure he's on his mark. This was a genuine gaffe by Harold Ramis, and he and Ivan Reitman decided to keep it in because it showed how awkward the inexperienced Ghostbusters were in front of the camera.
In Ghost Rider, the first scene with Nicolas Cage as Johnny Blaze, a professional daredevil, has him failing a jump. During the fall, the front wheel of his motorcycle smashes into his helmet, breaking the visor of the helmet. This was not intentional, and the stuntman really did take a tire to the face. However, when the stuntman saw the footage of the crash, he thought it looked good, so they decided to leave it intact.
Ripcord's helmet flashing holographic pictures of Scarlett in G.I. Joe: The Rise of Cobra. Originally, he was meant to be responding to the amount a virtual controls showing up in the HUD, but when it got to actually including the display in the visual effects department, they thought it'd be more amusing if it had her face popping up.
The Right-Hand Cat in the opening scene of The Godfather was not in the script. It was just some random stray cat that Marlon Brando befriended, and argued Coppola into letting him work it into the scene. And it works. In the same movie Lenny Montana, playing Don Corleone's henchman Luca Brasi, actually flubbed the line where he congratulates Don Corleone on his daughter's wedding. Coppola liked it, and inserted a scene earlier in the film, where Brasi is rehearsing his congratulation.
"Don Corleone, I am honored and grateful that you have invited me to your daughter...'s wedding... on the day of your daughter's wedding. And I hope their first child be a masculine child. I pledge my ever-ending loyalty."
Legend has it that Lenny Montana (who worked for the Colombo crime family) was one of the thugs sent down to the set to see how the movie portrayed the Mafia, and whether changes needed to be made to the script; one of their demands, for example, was that the word "Mafia" not be used. The actor playing Brasi had had a stroke, they needed a replacement, and Lenny got the part. He was a big fan of Marlon Brando, and flubbed the lines because he was so nervous about meeting him.
Clemenza's now famous "Leave the gun, take the cannoli" line was a half-improvisation by Richard Castellano; the gun part was in the script, the cannoli part was not.
In the final battle of Terror Of Mechagodzilla, there's a moment when (due to a nearby explosion) Godzilla's back-spikes catch fire. You can bet they left that shot in.
In the first American Godzilla (1998), there's an establishing shot of Manhattan from the south, in which an ominous bolt of lightning strikes one of the Twin Towers. It's totally real.
In the opening number of Golddiggers Of 1933, Ginger Rogers sings "We're in the Money. In between takes, the director heard Ginger joking around speaking fluent Pig Latin. He then decided to put in a part where the camera closes in tight on Ginger as she sings a verse of the song in Pig Latin.
During the chase sequence in Gone in Sixty Seconds (1974), the Mach 1 Mustang driven by the protagonist was accidentally clipped from the rear by another car, causing it to spin out of control and collide with a lamp post. The collision was kept in the film for dramatic effect.
In The Great Escape during the Fourth of July scene, Goff's line "No taxation without representation" was an ad-lib, causing Steve McQueen to do a double-take.
There is a scene in Harry Potter and the Chamber of Secrets where Draco Malfoy asks Harry, disguised as Goyle, why he was wearing glasses, as Harry had forgotten to take them off. When Harry replies that he had been reading, the script originally only had Malfoy stare him down skeptically. Tom Felton decided to mischievously add the line "I didn't know you could read," instead, and it was kept.
In the same film, actor Jamie Waylett didn't realize his character wasn't supposed to participate in the Slow Clap at the end, so he stood up and was pulled down again by Tom Felton. They kept it.
Jason Isaacs improvised two lines in the film: the first, as he was leaving Dumbledore's office and felt it was un-Lucius-like to let Dumbledore get the last word, he turned to Daniel and sneered "Let us hope that Mr. Potter will always be around to save the day." Daniel similarly ad-libbed Harry's reply: "Don't worry. I will be." The second ad-lib was Lucius' curse cut short by Dobby; the script didn't mention any specific spell so he just recalled from memory the "Avada Kedavra" curse, which led to some fans wondering how Lucius thought he could be using an unforgivable curse on Harry outside of Dumbledore's office.
In an interview, Isaacs revealed that a scene in the second film, where he kicks Dobby and then knocks him down with his cane, was also ad-libbed by him. Christoph Columbus first thought he had tripped, but after hearing the explanation, he was delighted.
When Hermione meets Harry in the first film, she says "Holy cricket, you're Harry Potter!" According to Emma Watson, she ad-libbed the "holy cricket" part and Chris Columbus thought it was hilarious.
In the fifth film, the Trio cracks up at the end of the scene when Harry tells Ron and Hermione about his kiss with Cho. This was an instance of Corpsing, which David Yates left in because he thought it hit the right tone.
Voldemort's memetacular hug of Draco was improvised by Ralph Fiennes. According to Tom Felton, they did over twenty takes of that scene and Ralph Fiennes only did the hug once. They used it, obviously.
More of a funny mistake than an intentional improvisation, the film Hot Fuzz includes a scene where Simon Skinner, Timothy Dalton's intentionally-played up bad guy who is in fact merely a Disc One Final Boss raises his glass and for a split second looks right down the barrel of the camera. Director Edgar Wright decided to leave the outtake in, and even timed the sound of a bell in the background to accompany it. Additionally, Danny Butterman's "I'm not made of eyes" was ad-libbed by the actor. Similarly, the first scene where Dalton's character gets introduced (when the two are jogging) was going to be reshot, because Dalton kept unintentionally pushing Pegg out of frame. They decided to keep it in, as they felt it fit Skinner's character.
In The Hours, when Meryl Streep goes to the sink and turns it on, the faucet explodes and shoots water up into the air; Meryl just went with it, and they kept the take.
A unique variant of "throw it in" occurred during production of The Hunger Games. Originally, Donald Sutherland's character, President Snow, was only planned as a cameo, in keeping with the books in which Snow doesn't become a major character until later. However, Sutherland sent director Gary Ross a very detailed letter in which the veteran actor discussed Snow's frame of mind and his motivations. The letter (which Sutherland reads as a bonus feature on the DVD/Blu-ray release) motivated Ross - who happened to also be one of the film's writers - to craft several additional scenes involving President Snow, giving the character a much greater presence in the film.
The final scene from I Am a Fugitive from a Chain Gang had Paul Muni disappear into darkness as he said his final line "I steal," thanks to the lights being turned off a bit too early. Everyone agreed it was the perfect touch to end the film on.
In the French movie Il y a des jours et des lunes, a priest who acts in amateur plays is at one point complimented on his acting skills. The actor playing the priest was supposed to answer with a joking "You're telling me you want to be my agent?" but flubbed the line into "You're telling me you want to be my apostle?" When he realized, he started laughing hysterically but tried to stay in character by apologizing and talking about Freudian slips and blasphemy before repeating the real line. The director decided to keep it because the slip was just too good.
In Inception the infamous "You mustn't be afraid to dream a little bigger, darling" line was said as a joke, but Nolan liked it and decided to throw it in. Specifically, the majority of the line was in the script; the "darling" part was not.
A slightly more plausible version says that Steven Spielberg said sarcastically to Ford that the only way the scene could get shortened is if he just shot the guy. The crew began laughing at the idea and they worked it out.
Another version holds that Ford and most of the crew had gotten sick, and this was the last scene they needed to film in this location. Ford goes to Spielberg and says "Look, Indy wants to save the girl, right? He doesn't have time for this, so why not have Indy shoot the fucker?" And so he did.
According to the Making of Indiana Jones book, they did manage to shoot a completed fight scene with the swordsman. There were two versions during the editing process. One cut with the fight with the Arab and one with Indy just shooting him, with George Lucas preferring the former and Spielberg the latter. They left it to a test screening to decide which to use. Indy shooting got the biggest laugh and was kept in.
In Indiana Jones and the Last Crusade, in response to Indy asking his father how he knew Elsa was a Nazi, Jones Sr. simply replies "She talks in her sleep." Sean Connery actually ad-libbed that line, and it was kept since it made the entire crew burst into laughter.
The "Put on a phat beat for me to beat my buddy's ass to." line in the Iron Man 2party scene. It's pretty obvious (as Robert Downey Jr. immediately cracks up on camera), but pretty hilarious as well. (And it's in character as Tony Stark is supposed to be drunk.)
Apparently Robert Downey, Jr. improvised a lot of lines in both Iron Man films, particularly his bits with Pepper.
The whole cast did when Favreau was directing; most of the they had beats he wanted the actors to hit, but left the actual dialog to them. That why both films have multiple occurrences of people talking over each other, because both actors are tying to say their lines at the same time.
In the classic Russian holiday film The Irony of Fate, or Enjoy Your Bath!, the Love Interest's fiancé Ippolit is thrown into a shower fully clothed in order to sober him up. After the water is turned on, Ippolit says "Oh, hot water, very nice!" Apparently, it took a while for Russian showers to warm up, so the actor was surprised when he got doused in hot water instead of cold. The director loved the unintentional ad-lib and kept it.
Thomas Mitchell, the actor playing the drunken uncle is accompanied by a loud crash on one of his exits; the noise was actually caused by a grip tripping over a prop table and scattering its contents, but the timing was so serendipitous that director Frank Capra decided to use the take anyway. They were going to re-take it, but Mitchell shouted "I'm alright! I'm aallllll... right." That saved the take, as it made it look like he'd just done an off-screen collision with a garbage can. The grip thought he would be fired on the spot. Instead, Capra gave him a $10 bonus for "improving the audio quality of the movie."
There was much more dialogue in the scene where George and Mary are both talking to Sam over the phone — but that long kiss was so much better than the dialogue Capra scripted that it got used instead. Technically, that might be "Throw It Out" as much as "Throw It In".
In the building and loan panic scene, the woman asking for $17.50 wasn't originally in the script. Capra fed the actress the line before shooting without telling James Stewart, so he could seem genuinely surprised when she said it. He was, and the grateful kiss afterward was an in-character ad-lib on Stewart's part.
Live and Let Die - during the shooting of the boat chase, a boat that Mr. Big's mook is piloting didn't make the jump over the road and smashed through a police car - it was quite in line with the comic tone running through the movie.
The Spy Who Loved Me. In the scene where Major Amasova couldn't drive stick, Barbara Bach, Anya's actress, actually couldn't drive stick: Moore's snarky responses were unscripted!
In the scene where Renard is told by Bond that Elektra is dead the actor Robert Carlyle's make up is actually slipping, but the director thought his performance so powerful that he kept the shot in. It's actually easy to mistake the slipping make up for tears, such is how it comes across!
Pierce Brosnan ad-libbed the bit where he adjusts his tie during the boat chase at the beginning of the movie.
The Casino Royale remake had the iconic scene of Daniel Craig walking out of the water showing off his well muscled physique as well as doubling as a Shout-Out to the very first Bond film Dr. No. In reality, the shot was supposed to be of Craig swimming to shore. However, he hit a sand bar, forcing him to stand up. The resulting shot was too good not to include and cemented Craig's status as a Memetic Sex God as well as win over some skeptical fans.
Of the two shooting stars that appear during the shark's night attack on the boat, the first one was apparently real and kept in due to being a real one-in-a-million shot.
When the barrel whips over the front of the boat and knocks Brody's glasses off, it wasn't meant to get that close to Roy Scheider, and his reaction was at least partly natural.
The footage of the live shark thrashing around in the cables supporting the cage was captured when the animal accidentally got stuck there. This contributed to Hooper surviving as legend has it the dwarf actor they were using for purposes of scale refused to get back into the cage afterwards!
During the first take for Quint's Indianapolis speech, Robert Shaw was extremely drunk. They reshot the scene with him sober, but Shaw's performance in the first take fit so well with Quint's character, that the crew actually edited cuts from both takes into the scene.
In John Carpenter's Vampires, John Carpenter got along with James Woods, a notoriously hard to work with actor, by allowing him to to ad-lib as long as he did at least one take strictly by the script. According to the DVD commentary, a lot of the ad-libs made it to the final cut.
Jerry O'Connell beatboxing in Kangaroo Jack. According to the DVD commentary, Bruckheimer heard O'Connell and Anderson beatboxing on set and asked O'Connell if he could do that "spit-rap thing" in the movie. Extra funny because people assume it isn't really O'Connell doing it in the movie since both the actor and character are pretty white.
In Kick-Ass, the entire bazooka subplot was improvised in the course of filming.
In Kill Bill, Daryl Hannah went off-script when she started screaming and flailing around in the trailer after the Bride vs. Elle battle. Apparently, Tarantino liked it.
The rather strange opening sequence was filmed on the second unit as a joke, and then the director decided to use it for real.
The scene where a crowd fails to cheer until one of the main characters does. This happened because the extras were all Czech, didn't understand English, and at first actually didn't realize they were supposed to cheer.
In one shot where in response to how to beat William, Adhemar's page says that "With a lance, on a horse..." "...he's unbeatable." This shot was actually the result of a cameraman not realizing he was supposed to be doing a close-up until right as the action began.
In Kramer vs. Kramer, the last scene of the movie where Joanna (Meryl Streep) asks Ted Kramer (Dustin Hoffman) "How do I look" and he replies "You look terrific" took place before the filming was supposed to begin, apparently Robert Benton liked it more than the original scene, and left it in.
At the very end of Martin Scorsese's film adaptation of The Last Temptation of Christ, in a scene depicting Jesus' Crucifixion, the film image suddenly dissolves and then goes stark white, as if there were a sudden light leak in the camera while they were filming. Turns out that that's exactly what DID happen - something had gone screwy with the camera while they were filming the scene, and no one noticed until they reviewed the footage later. But since it happened at precisely the point of Jesus' death in the film, Scorsese kept it in.
In The Fellowship of the Ring, Ian McKellenaccidentally hit his head on the ceiling while entering Bilbo's residence. This was kept in the final cut as a joke. Bumping the hanging lanterns was scripted. His quick turn to his left, apparently to avoid the hanging lanterns... results in a painful whack into the low ceiling-strut as he does so, and this was not scripted. Ow...
Also in the first film, during the final fight between Aragorn and the leader of the Uruk-Hai hunting party, the actor in the Uruk makeup was supposed to fake a head-butt to Viggo Mortensen. But the makeup evidently made it difficult for him to judge the distance, and ended up giving Mortensen a very real head-butt. The move, and Mortensen's very real pain, made it into the film if it hit him point first.
From the same fight scene, Makaore (the actor playing the Uruk) throws a knife at Mortensen. The script called for him to throw it and miss, but he actually threw it straight at Mortensen... and Mortensen deflected it with his sword, a completely unplanned move so cool that it would be hard to believe it wasn't either practiced, or special effects. It should be noted though, that it was a real knife, meaning that if Mortensen hadn't successfully deflected the knife, it would've gone straight into his face.
In The Two Towers, Aragorn comes upon a scene that seems to indicate that two of the other characters are dead. On the extended DVD, they show several takes of him snarling in helpless anger as he kicks an Orcish helmet. Then, they try one more take... and he collapses to his knees, screaming in fury, grief — and pain. Viggo Mortensen broke two toes kicking the helmet and decided to use it. They finished the shot, and then brought the medics in.
In the scene where Éowyn runs onto the terrace after her confrontation with Wormtongue and stares out at the landscape, a banner suddenly tears off its pole and blows away. That was not intended; genuine high winds were blowing everything around violently. But the symbolism was so provocative and moving it was decided to keep the shot, and a follow-up scene was later filmed showing the banner landing near Aragorn as he rides up to the base of the hill.
Hell, Viggo had it so bad it's a wonder he didn't make a national complaint or get badly hurt or killed. In one scene, Aragorn is floating down river, face down with FULL ARMOR ON. Viggo almost drowned. All of Viggo's "happy" accidents is what cracked used on their top six list in the web originals section.
During filming of the Battle of Helms Deep, some of the stuntmen playing Uruk-Hai relieved their boredom between takes by making a game of tapping their spears against the ground in unison. This gave Peter Jackson the idea to have all 10,000 Uruk-Hai do it as they arrived as an intimidation method.
Professor Tolkein, as an Anglo-Saxon scholar would have approved. Banging weapons is a centuries old idea- it intimidates the enemy, keeps your morale up, and helps keep a tight shield wall in step. You can still see riot police do the same thing today.
One specific instance is described on the special edition extra content, where they were about to shoot the Elves counter-attacking against the Uruks coming through the breach in the deeping wall. The actors playing the Elves were a little timid (probably their first shoot as Elvish warriors). At the time, there was a large group of stuntmen in Uruk costumes standing across the way, who began stomping their feet, beating their weapons against their chests, calling them names and even making obscene gestures at them, as well as the Maori haka. This quickly got the Elvish stunt-doubles riled up, and they in turn began posturing and drawing imaginary arrows at the Uruks. Then suddenly, to everyone's surprise, the director yelled "Cut!". Part of the footage got into the film, though they had to cut out parts with gestures and exclamations that were not... native to Middle-Earth.
In The Fellowship of the Ring, as Frodo is sailing away while Sam resolutely attempts to chase him down, Sam suddenly slips and falls into the river, and does not come back up. This is because Sean Astin had stepped on a particularly sharp rock and cut his foot; he's gone down for real. They kept it in and had Frodo come back to save Sam from drowning.
In Mad Max 2: The Road Warrior, one stunt involved a motorcycle hitting an embankment, whereupon the stuntman was to flip over the handlebars and land on his back on padding in a standard stunt move. The stunt went wrong, the stuntman flipped head-over-heels two or three times, and wound up breaking both legs. The scene made it into the final film without reshooting, because it was that awesome.
In the original 1962 The Manchurian Candidate, the scene where Major Marco overrides Sgt. Shaw's brainwashing by showing him an entire deck of queen of diamonds has Major Marco slightly out of focus. Director John Frankenheimer later claimed to have heard theories this was done intentionally to show Marco from Shaw's hazy, brainwashed point of view. In fact, it was a technical glitch. They had tried to reshoot the scene with the camera in focus, but according to Frankenheimer, Frank Sinatra's performance was at its best in the first, out-of-focus take, and it deteriorated in take after take. In the end, they decided to use the take with the best performance, out of focus or no.
"It just be rainin' black people in New York!", said by Edwards as he drops onto a tour bus, ad-libbed by Will Smith.
Another Will Smith ad-lib made it into the sequel, when Agent J first shows Agent K the car's new "autopilot," a life-sized human model that pops out of the steering column;
Agent K: Does this come standard?
Agent J: Actually, it came with a black dude, but he kept getting pulled over.
Almost all of Tommy Lee Jones' lines in the first film were ad-libbed. Jones hated his character's lines, so he made up his own. Will Smith is genuinely confused half the time.
A much-disputed, yet nonetheless famous instance, is in Midnight Cowboy. One of the producers insists that the cab that prompted Hoffman's now-famous "I'm walkin' here, I'm walkin' here!" was driven by an actor, and that the production team was told to make the near-hit appear to be ad-libbed. However, when on Inside The Actors Studio, Hoffman claimed that he and Voight were not supposed to be nearly hit by any traffic, even from paid drivers, and that his reaction was in lieu of "We're filming a movie here!"
Of course, if Dustin Hoffman is to be believed, he ad libbed nearly every line he ever uttered in a film, so one might take his version with a grain of salt.
Also, allegedly the scene was filmed with a hidden camera because the production staff couldn't get permits to film in New York and they had done over 15 takes to get the street crossing scene right, which would support the idea that the line was an ad lib.
Ivan Ooze's rant about the horrible things inflicted on humanity he missed out on: "The Black Plague! The Spanish Inquisition! The Brady Bunch Reunion!" That last one was ad-libbed by Paul Freeman, which everyone on the set loved.
Johnny Yong Bosch also ad-libbed his disappointment about his ninja animal being the frog, and the cast and crew liked it well enough that it was kept.
It is even funnier because it broke the logic. That part was quite serious, until that line.
In Monty Python's Life of Brian, when Brian is telling everyone that they are all individuals, and they mindlessly repeat it, the one guy who goes "I'm not!" is an extra who just threw that out there on the spur of the moment. He got a pay raise to speaking actor.
The "He hasn't got shit all over him" line was improvised.
John Cleese has an improvised moment in the Burn the Witch! scene; when asked why witches burn, the crowd is stumped. Cleese has the next line: "Because they're made of wood?" However, according to the DVD commentary with Eric Idle, he experimented with the timing between the question and the answer, even going so far as to start answering and then go back to thinking. Watch Eric Idle in this scene; towards the end of the pause he's biting down on his scythe to keep from laughing.
The line, "There are those who call me... Tim?" According to some versions of the story, the Enchanter did have a more appropriately mystical name, but Cleese forgot it while shooting.
In general however, the Python troupe rarely used ad-libbing.
In My Big Fat Greek Wedding, Toula's aunt, who goes up to Ian (Toula's fiancé) to ask him something, randomly says "Let me touch your hair", and begins to massage it. This was an ad lib on Andrea Martin's part, as she'd forgotten her line.
The scene from Mystery Men in which a burning trash can suddenly flares up behind the Spleen who jumps in surprise, then sheepishly says "Excuse me" to the other characters was a total improvisation on the part of Paul Reubens. Apparently, one of the workmen on the set didn't know the garbage can was a prop that would be later set on fire, and had thrown a disposable lighter into it.
In Napoleon Dynamite, there is a scene where Napoleon attempts to hop a fence but ends up falling over onto the other side. Jon Heder actually fell while taking this shot, and the makers decided to keep it.
Also, the scene where Kip is giving the demonstration with the van and the dinnerware. Originally, he was to roll over it, and the plastic would deform but reshape itself, to which Kip was to say, "Pretty cool." However, the plastic was unable to comply due to the laws of physics, and that is what is seen in the film. It turns out to be actually funnier.
Robert Englund improvised quite a few of Freddy Krueger's one-liners, but the best-known example happened in A Nightmare on Elm Street 3: Dream Warriors, in a scene where Freddy emerged from a television set and killed a girl by smashing her head into it. The scripted line was "This is it, your big break in TV!" which Englund said on the first take. When the director went for an alternate angled shot however, Englund changed the line to "Welcome to Prime Time, bitch!" The different camera angle made it easy to edit the two lines together, and it became probably Freddy's defining one-liner. According to The Other Wiki, the line was originally "You're on TV now, girl!"
The PC LOAD LETTER scene from Office Space. The machine beeped with an actual error message after Michael put paper in it which threw off the actor's line, and the entire thing was improvised to try and salvage the scene.
In The Outsiders, the scene where Dallas falls out of his chair while flirting with Cherry at the movies was an accident. You can see C. Thomas Howell briefly look at the camera.
During the carrying-his-cross scene Jim Caviezel dislocated his shoulder when he collapsed and the cross fell on him. He insisted that the take be kept in the final film, so that the pain Jesus was supposed to be experiencing would seem more real.
During the flogging scene, Caviezel was wearing a protective guard on his back while being whipped, but on the last blow the guard slipped and he was whipped for real.
In Curse of the Black Pearl, Jack's statement that he used "human hair — from my back" was an ad-lib. You can see Bloom trying not to laugh, and McNally chuckling in the movie. The commentary states that they initially tried to edit it out, but they found that the line lost something without it, so they threw it in.
In the trailer for Pirates of the Caribbean: Dead Man's Chest, Will says to the other characters "I'm not leaving without Jack!" while getting on the Black Pearl. When he sees that Jack is on the other side of the beach getting chased by a large group of natives, he says, "Never mind, let's go!" This line didn't make it into the movie (because they did indeed wait for Jack, more or less), but its creation was actually from a blooper where Bloom flubbed his line, and "Never mind, let's go!" was an effort to just keep going and say the line again without hesitation. This fact can be found on the Dead Man's Chest DVD commentary.
Jack's line "I've got a jar of dirt!" was also unscripted.
All of Jack's jokes about Will supposedly being a eunuch were ad-libbed by Johnny Depp. Through the creators' approval of the first, he continued.
According to sources, pretty much the whole of the character in the first film was a Throw It In, as originally, Jack was supposed to be more of a background character, a sort of sidekick to William, but Depp decided to go with his now famous zany approach to the character, that for a while, had the director and the executives not too pleased with him. Depp told them "trust me, or fire me." He still improvised a lot in the second and third films, but in the first film, it led to a completely different character.
Literally the only explanation for the bulk of Pocket Ninjas; many of the things going on, especially during supposed fight scenes, only make sense if you assume the actors were clowning around without realizing the cameras were rolling and the director (who may or may not have been drunk and high at the same time) decided that that was exactly what he wanted in his movie.
Christopher Walken's trick shot in Poolhall Junkies was accidentally filmed. As he was being taught how to perform it, he tried for the first time as practice and sunk the shot. They were filming, at his request, since he was afraid that he would be unable to sink the ball in any following take. If you watch you can see all the actors in the scene gasp and begin to laugh, even Walken looks surprised.
According to an interview with Uwe Boll, the infamous Dave Foley full frontal nude scene in Postal was a result of this: Foley was sitting down wearing only a robe and Boll had merely instructed him to stand up. Apparently, neither of them anticipated that the robe would suddenly open. Boll found this so funny that he had to leave it in.
When Kevin Kline's character in A Prairie Home Companion opens the bottle of champagne, it was director Robert Altman he hit in the forehead with the flying cork. Kline's "Sorry!" was unscripted.
In Pretty Woman, there's one scene where Richard Gere is showing Julia Roberts' character a very expensive necklace in its open box. The original scene as written simply required Roberts to tentatively touch the necklace and say it was beautiful. Instead, as Roberts touched the necklace, Gere clapped the box's lid down on Roberts' hand, scaring the hell out of her and making her laugh. Because of the way it looked on film, the take was left in — watch where Roberts turns to when the joke is played on her — she's looking offscreen at the crew, not anywhere near a camera.
While filming, Anne Hathaway slipped and fell on her butt while filming a scene on some bleachers. Garry Marshall put that cut in the movie as her character was a Dojikko anyway. You'll notice Heather Matarazzo briefly slips out of character when this happens. Hathaway did this quite a bit during the film as when Mia accidentally sets fire to a man's arm at the state dinner, the fire was meant to go out in the ice bucket but Hathaway panicked and threw the glass of water on him and it was also her idea that the brush break in her hair when Mia is getting her makeover.
The entire Clarisse and Joseph romance was actually a case of this. It was not scripted at all and the dance scene and affection between the two was added by their actors.
Miller's monologue about time travel, flying saucers, the Mayans and plates of shrimp in Repo Man was written for the screen tests, as was Lite's monologue while he and Otto are breaking into the red Corvette. Both were added in when the filmmakers realized how funny they were and the former became a key part of the film's ending.
In Requiem for a Dream, the shot of Ellen Burstyn delivering a tearjerking monologue of why her character wants so badly to lose weight and be on television is slightly askew. When director Darren Aronofsky confronted his cinematographer who was operating the camera, he admitted he had let the camera slip because his crying had actually fogged up the lens. Aronofsky left this shot in the film.
Return of the Living Dead has one scene toward the end where Frank immolates himself in the oven. This is because James Karen didn't want to shoot his final scene in the cold rain and instead suggested that Frank commit suicide because he's a nice guy and didn't want to hurt anybody.
In Return of the Pink Panther, Catherine Schell can be seen breaking into laughter at some of the antics of Peter Sellers. The two scenes in question are when Insp. Clouseau impersonates a telephone repairman, and later when Clouseau meets her in a restaurant and pretends to be a lounge lizard; in this latter example the scene ends with Schell choking on her drink. It's been said Schell's laughter (and the choking) were outtake-worthy moments that the director decided to keep; Schell has claimed they were scripted.
In Road To Morocco, Bob Hope and Bing Crosby are stranded in the desert when they find a convenient camel. In mid-line, Bob gets spit in the eye by the camel, and Bing laughs "Ho ho, good boy!" They pretty much had to keep that in.
During a car chase, a hubcap comes loose and rolls almost directly at the camera. Paul Verhoeven left it in since it looked very cool.
In the scene where Robocop faces two would-be rapists, the groin shot was a last-minute addition. Originally Robocop was to just shoot the hostage taker in the upper body, just missing the woman he was holding. When they noticed that the way the actress posed herself when struggling would allow a clean shot through her skirt.
In the original Rocky, loan shark Tony Gazzo is talking with Rocky about Rocky not breaking the thumbs of one of his clients, when he pulls out an inhaler in mid-sentence and uses it. This unscripted action happened because the man playing Gazzo actually had an asthma attack at that moment, and the director liked how it made the scene more authentic.
The shot in which Rocky runs through through the market and someone throws him an apple was actually a member of the public trying to hit Sylvester Stallone as he was filming the scene, unbeknown to Stallone who at the time thought it was part of the filming. The director however liked the shot and kept it in the scene.
According to multiple interviews by Gregory Peck, the famous scene in Roman Holiday in which he pretended to have his hand bitten off by the Mouth of Truth was ad-libbed by him, with only the director being aware of it in advance. Audrey Hepburn's scream and her relief laughter were genuine reactions. According to Peck, he borrowed the joke from Red Skelton.
In the roulette scene of Lola Rennt (titled Run Lola Run in English), an initial take was filmed of the wheel spinning and the ball being dropped, with the intention of later editing it together with a staged shot of the ball landing on twenty to complete the scene and win her the money she needed. The ball landed on twenty in the first take.
Matt Damon's story about his brothers in Saving Private Ryan was ad-libbed. Tom Hanks' gaze flits off-camera for a second, then a slow nod. Is that "Captain Miller hears you, Private Ryan", or "Tom Hanks hears you, Steven Spielberg: run with it"?
In Scott Pilgrim vs. the World, Wallace bursts into the apartment drunk and throws his keys at Scott's head. The actor did this as a joke, but Edgar Wright loved it so much that he kept it in.
The bit in Scream (1996) where Billy goes to give Stu the phone, but it slips out of his hand, hitting Stu was an accident. Matthew Lillard screamed out "You fuckin' hit me with the phone, dick!" The moment made director Wes Craven laugh so hard, he chose to keep it in.
Also, in the climax when Billy is attacked with an umbrella, his screams are real as the stuntman had hit a implanted wire in his chest. The wire was from heart surgery he got as a kid and touching it causes him immense pain.
Also from Scream 3 (and likewise in the climax), when Roman is searching for Sid and tries to find her with his phone only for her to beat him to the punch, distracting him. She pops up from behind a bar and stabs him in the shoulder with an ice pick. The scream from actor Scott Foley is real as she had missed the pads and actually struck flesh.
At the end of Sea Of Love Al Pacino bumps hard into an approaching passerby while walk&talking. He gets hit so hard, that he's actually knocked back a few steps, yet doesn't even so much as blink, and fluently continues his speech. That wasn't scripted, in fact the guy wasn't even an extra. In the dvd-commentary, the director explained that they couldn't close off the whole location, it being a public street in New York, and that the pedestrian was real. It's realistic, because he's trying to convince the woman he loves to give him a second chance, so it's understandable that his character completely ignores it. Plus it's New York, people who live there probably don't even notice anymore.
In Serenity Mal's "Faster! Faster would be better!" is sucha Whedon line. It turns out it was ad-libbed, when Nathan Fillion was asked just to "say something Mal would say."
Word of God on Brad Silberling's director commentary for A Series of Unfortunate Events states that Jim Carrey ad-libbed quite a few of his lines during practice runs. His practice lines damn near perfectly added to the scene's mood almost every time and were memorable even when they didn't, so Brad shrugged it off and said, "Eh, what the heck." Thus, almost all of his best lines in the movie were actually cooked up during practice runs. Overlaps with Harpo Does Something Funny, because anyone willing to cast Jim Carrey knows he can make a scene absolutely perfect if you don't try to order him around too much.
Hannibal Lecter's famous hissing was improvised; indeed, was enough of a joke that the actors didn't expect it to be kept in the film. You'll notice that there's a nice long pause between "A nice Chianti" and the hiss, presumably so that it could be cut without damaging the line. The director decided it struck the right tone, after noticing Jodie Foster was quite genuinely creeped out.
Hopkins improvised the bit where he mocks Clarice's accent during his Hannibal Lecture; this overlaps with Enforced Method Acting because he did not inform Foster that he was going to do this, so the surprise on her face is genuine.
Billy Wilder and I. A. L. Diamond struggled for days with the final dialogue between Jerry and Osgood in Some Like It Hot, trying to think of an appropriate answer from Osgood when Jerry reveals he's a man. Unable to think of anything funny, they gave up and had Osgood say "Nobody's perfect." This has gone down in film history as one of the funniest punchlines and film endings ever. Billy Wilder even used the sentence as the title for his own autobiography. It's also on his gravestone.
In The Sound of Music, Julie Andrews tripping at the end of "I Have Confidence" wasn't scripted, but was so perfectly in line with her character that it was left in.
Nearly all of Spaceballs was co-written by Mel Brooks, but the lines for the scene where Dark Helmet plays with his action figures? They were ad-libbed by Rick Moranis, who plays Helmet in the movie.
Harold Lloyd's silent 1928 comedy Speedy climaxes with a high-speed chase through the streets of NYC by a horse-drawn trolley. At one point during location shooting for the scene, the trolley crashed into a pillar holding up an elevated railroad platform. This was kept in the final film.
In Spider-Man 3, Topher Grace ad-libbed the quip, "My Spider-Sense is tingling, If You Know What I Mean." Apparently, he also added all sorts of jokes that didn't make it in, such as a needlessly creepy "I've just upgraded from a vanilla to a strawberry!" addressed at the red-headed Mary Jane.
In the 2002 caper movie Stark Raving Mad, when Ben is knocking out the nightclub owner with a convenient bottle, they had several takes where the Soft Glass bottle didn't actually break — but the actors were so dedicated to selling every take that they just had Ben shatter the bottle and knock the guy out with the second blow.
In Star Trek II: The Wrath of Khan, the design of the Starship Reliant was not supposed to look like it did in the movie: the visual effects team sent the design sketch to producer Harve Bennett for approval, and he signed off on the sketch upside-down. The vis-effects people realised that it actually looked better upside-down, and was more distinguished from the Enterprise with the nacelles angled down rather than up, so they built the model according to the "upside-down" view. That design of starship has been used in subsequent movies and shows.
For his part, Shatner confirms in his book Star Trek Memories that he did, in fact, just miss the chair and fall on his ass by accident.
In Star Trek 2009 McCoy's "All I've got left are my bones" line is an ad lib as is Scotty's "can I get a towel." You can see Spock's lips twitch after that one since he's trying not to laugh. In a previous take of it he did laugh.
One morning in 1986, San Francisco native Layla Sarakalo discovered her car had been towed because the public parking space had been made available for a film crew truck. She figured the best way to get money to pay the towing fee was to work on the film that day as an extra. She managed to get hired that day to join the other extras. She was feeling a bit nervous, having never worked on a film before, so the other extras told her to "act naturally". When she got stopped by a "Russian" asking her how to get to the naval base in Alameda where the "nuclear wessels are", she naturally responded, "Ooh, I don't know if I know the answer to that. I think it's across the Bay. In Alameda." The director, Leonard Nimoy, loved that moment so much, he threw it in and it became one of the most frequently broadcast clips from Star Trek IV: The Voyage Home.
Although the way Leonard Nimoy tells it in his autobiography I Am Spock, that shot was really just Walter Koenig and Nichelle Nichols accosting random passers-by and asking them about "nuclear wessels". Sarakalo was apparently just passing by, listened, and gave the instructions, and it's that shot which remains in the film — they made a contract with her afterwards. One genuine ad-lib in there is the impassive police officer being asked questions: he was a genuine police officer there to provide security, and his reactions were just recorded on film and got in there.
The Empire Strikes Back: Han's reply to Princess Leia saying that she loves him was originally supposed to be "I love you too," but Ford ad-libbed "I know," because he felt it to be more like the character. The director Irvin Kershner said they had ran through several different lines because the "I love you too" line felt too lovey-dovey for someone like the Loveable Rogue Han Solo, so eventually he just asked Ford to say what felt natural. Kershner loved the result — "I know" was the final take, at least before lunch — but George Lucas was afraid it was Mood Whiplash. Lucas was proven right when airing for a test audience — but the audience also felt the line was classic Han Solo, so he agreed to leave it in.
In A New Hope when Luke and Han were rescuing Leia disguised as stormtroopers and Han was forced to respond via radio to their commander, Ford intentionally did not memorize his lines and only briefly looked at what he was supposed to say. So while the scene as written is supposed to be Han improvising, "We're all fine here, thank you... How are you?" Ford played it panicked and grimacing at the last line. Again, it was good enough to keep, and provides a great bit of comic relief in the middle of a tense sequence. Harrison Ford just seems to be a magnet for these.
He pulled a similar stunt while filming The Fugitive (see details below).
Apparently Luke's remark "I can't see a thing in this helmet!" regarding his Stormtrooper disguise, was made by Mark Hamill after he thought the cameras had stopped rolling. This led to another Throw It In moment, when the Stormtrooper smacks his head on the door. A moment that's so famous, recent DVD releases apparently add a "thud" sound effect when it happens. You just can't get the help these days, can you Lord Vader?
Jango Fett gets banged slightly by a descending ship door in Attack of the Clones in homage of the above, even though that scene is done in CGI.
One explanation for the difference in dialogue quality between the original trilogy movies (especially A New Hope and The Empire Strikes Back) and the new trilogy is that the dialogue was either ad-libbed or "improved" on by either Harrison Ford and the other actors more or less on the spot or the screenwriters who collaborated on the script.
The improvisation/script alteration was remarked on by Mark Hamill in an interview around the time of the film's release. Apparently Harrison Ford had covered his script with alterations so that he could say the lines his way, and this encouraged Mark to alter some of his own lines. The "prisoner transfer from Cellblock 1138" was Mark's ad-lib (instead of a random string of numbers), which Lucas didn't originally want to use since it was a blatant Shout-Out to his earlier film, but got put in the final cut.
There is at least one instance of a "throw it in" in the new trilogy. Hayden Christensen and Natalie Portman improvised the dinner table scene in which Anakin mentions "Aggressive Negotiations". Apparently, Lucas didn't like the dialogue he had written for the scene, so he just told them to improvise. Portman later said that "it got inappropriate very quickly."
Also, in the same movie, the name of the benefactor of the Clone Army was intended to be Jedi Master Sido-Dias (who was a thinly-veiled disguise for Darth Sidious, and he was not even existent within the Jedi), but the scriptwriter made a typo (due to the d and f keys being right next to each other). George Lucas ultimately thought it was better, so they not only kept the typo as the actual name, they also rewrote the scene to reveal that Sifo Dyas had in fact died several years prior.
Anthony Daniels' entire performance as C-3PO in the original movie. Lucas figured he would just overdub the dialogue to get the characterization he wanted, then changed his mind after several people, including Mel Blanc, told him to go with Daniels' voice and interpretation.
A surprising amount of the jokes in Superbad were purely ad-libbed by the cast, usually until something was funny enough to cause the actors to break down laughing, and thusly added to the script.
While filming Sweet Sweetback's Baadasssss Song, Melvin Van Peebles obtained a permit to set a car on fire for a scene he wanted to shoot that weekend. Unfortunately, he got the permit on a Friday, and the city hadn't filed it by the time shooting was scheduled. He did the scene anyway and when the fire department showed up, he filmed it and left it in the finished movie.
In Taxi Driver, the scene where Scout dances with Iris while playing soul music was based on an improvisation Harvey Keitel came up with while rehearsing. He asked Martin Scorsese to include it in the movie, because it added so much to the character. Scorsese was reluctant to do that because the rest of the movie is from Travis Bickle's point of view, but once he realized Travis could be outside the apartment watching from his taxicab, the scene stayed.
In Teenage Mutant Ninja Turtles II, Michelangelo accidentally drops a piece of his pizza into the canister of anti-mutagen. You can tell this was unscripted by the way Mikey immediately looks up at the camera. Even funnier since he somehow has an Oh Crap expression on his (animatronic) face.
In Terminator 2: Judgment Day, Sarah Connor, after breaking out of her cell, ambushes an orderly by whacking him out and inflicting cuts in the process. This was not acted: Linda Hamilton actually inflicted the orderly's actor with the injury as revenge, because he went too easy on her when she was being restrained in an earlier scene (causing James Cameron to re-shoot that scene several times, and she had to fall to her knees on a hard tile floor each time). It was kept in the final cut.
The lady's bathroom scene in This Means War! was improv on the part of Reese Witherspoon... except the bit where she leans into the mirror and says "Has that been on my teeth the whole time?" They had filmed an eating scene earlier in the day, and she had a piece of schmutz stuck in one of her front teeth, and her reaction to seeing it was real. They used it in the film anyway.
While The Three Stooges were filming the train scenes for "Hold That Lion!", Curly Howard just happened to pay a visit to the set. Jules White saw an opportunity and improvised a scene with Moe, Larry, and Shemp harassing Curly as a snoring passenger. ◊
In Time Bandits, Katherine Helmond was supposed to play the role of the Mrs. Ogre (on the ship) in heavy prosthetic makeup, like her on-screen husband. She really wasn't looking forward to this, and suggested to Terry Gilliam and Michael Palin that it might be funnier to make the character a normal-looking woman who was actually stronger than her monstrous husband. To her relief, they went for it.
Leonardo DiCaprio telling Kate Winslet to get on the daybed in preparation for him sketching her nude portrait, saying "Get on the bed — errr, couch!" According to the director's commentary, the original line had no reference to a bed, but DiCaprio's nervous flubbing of the line seemed just too perfect to leave out.
When the ship is sinking and Rose comes to save Jack, when he jumps in the rising water, he says, "Shit, that's cold!" — apparently, unscripted. Rose goes "GAAAAASP!" in the same scene. This was also Enforced Method Acting, as the actors were told the tank of water would be warmer than it was.
When Jack and Rose are hanging on for dear life about two minutes before the ship sinks, Rose says "Jack, this is where we first met!". Complete ad-lib, but it makes the scene that much sadder.
Also an ad-lib was Jack's line as he is leaving the First Class dinner table: "Time for me to go back and row with the other slaves." James Cameron preferred it to the scripted line and left it in (one of the benefits of the director also being the script writer).
From Tom Jones, according to the IMDb: "Hugh Griffith was reportedly drunk through much of the production; the scene in which his horse falls on him was not planned, and many believed he was saved by virtue of his inebriated condition. The film incorporated every frame of footage before rescuers entered the frame to save him."
The crowd rushing the stage during the "Pinball Wizard" number in Tommy was not scripted, in one of the most spectacular "throw it ins" ever.
In Orson Welles's Touch of Evil, a scene featured a shot of Welles smoking. A piece of paper accidentally blew by in front of Welles. It was kept in at his request.
In the Transformers series Michael Bay is known for encouraging improvisation among the actors, which led to Steven Spielberg talking to the cast saying he would be looking at the dailies and saying "That's not in the script." Apparently in the first film, the reason Mikaela was mostly looking away from Sam while he was driving her home is because Shia LaBeouf improvised this long line of dialogue where she wouldn't recognize him because he lost 100 pounds at fat camp and the friends he met there have died from diabetes. Megan Fox could not keep a straight face.
It's also done occasionally to get more realism, such as the dialogue on the AWACS, which was improvised by the crew based on what they'd say in a combat situation like that (only without the giant robot scorpion...).
Word of God says that Curtis's slip and fall during the stripper dance wasn't scripted, and you can even see Arnold jumping out of the chair to see if she's alright. She instead jumped right back up and continued the dance, with Arnold sitting back down quickly. Luckily, all of this is perfectly in character (Harry would obviously be concerned about his wife, and then hastily attempt to maintain The Masquerade when the show goes on) and it ends up as one of the funniest scenes of the movie.
A scene when Arnold breaks a car window in frustration is also this — Tom Arnold's reaction is authentic, as Schwarzenegger broke the wrong (read: non-stunt) window without noticing.
Unknown Island When a horde of Ceratosaurs descend upon the heroes, who are throwing grenades at them, a little mistake made it in for the better. Because they were filming in mid-day in the desert with heavy rubber suits on. An explosion went off near one actor, who then promptly fainted from the heat. They kept it in the scene.
The lineup scene was scripted as a serious scene, but the actors didn't play it as such. One of the actors was constantly farting, which made it impossible for the other actors to stay serious. Bryan Singer was initially pissed off about it, but ended up using some of the funniest takes in the final film (so much that there's a take of everyone laughing). And in the scene where Redfoot the Fence flicks a cigarette into McManus' face, the reaction is entirely genuine: he was supposed to be aiming for the chest.
Fenster's status as The Unintelligible wasn't in the script. According to Benicio Del Toro, he came up with it (and Singer let him run with it), because Fenster was intended to be the Sacrificial Lion, and Del Toro figured that if he was going to be killed off early, he'd be memorable.
In Walk the Line, Joaquin Phoenix as Johnny Cash rips a sink out of a wall while portraying Cash as being affected by drugs. The sink-rip was not in the script.
Early in the film, Jennifer playfully pins David between her legs. According to Ally Sheedy, it was something done on a whim and didn't realize the sexual implications until after the scene had been included in the final film.
When David, Jennifer, and Falken were rushing to Norad on the Jeep, the Jeep wasn't supposed to crash, but it was kept since it added more dramatic tension.
Mc Kittrick asking David about his flight reservations to Paris wasn't in the original script. One of the crew saw the implication while filming and informed the director.
In The Warriors, the famous line "Warriors, come out to playyyyyay" was stated to have been improvised by David Patrick Kelly, inspired by a man who used to make fun of him in New York. The use of the beer bottles was also an improvisation. (The original intention had been to use dead pigeons, but it didn't happen.) Good thing too, since that's the one scene everyone remembers.
While filming White Heat, the crew ran into a problem. The scene takes place in the prison cafeteria, where CodyJarrett has just been informed of his mother's death. As written it was falling flat until Jimmy Cagney seated the two biggest extras on either side of himself and told the director to follow him with the camera no matter what.
The scene in Wild Hogs where John Travolta attempts to pull off a poor Clint Eastwood impression in the biker bar was completely improvised on the spot. "What the hell is wrong with you?" wasn't directed towards his character.
In The Wind and the Lion, during one of Teddy Roosevelt's monologues, a horse lies down and rolls. In the commentary, the director notes that most filmmakers would have reshot the scene, but he kept it in for verisimilitude.
Deliberately cultivated in The Wind That Shakes the Barley, where the actors were given very little rehearsal time, and much of the dialogue consists of them interrupting each other or stumbling over words.
X-Men: Reports suggest that the actual script of the movie didn't have Wolverine saying "bub" but Hugh Jackman, as a fan of the character, threw it in. Some reports state that he actually "threw it in" many, many times. They just only kept some of them. When Wolverine meet Professor X, he says "What do they call you — Wheels?" where the Wheels part was ad-libbed. The scripted line was "What do they call you — Baldie?"
Marty Feldman started covertly switching his prosthetic "Igor hump" from shoulder to shoulder between scenes, until someone on the production crew finally noticed, and a bit was added where Frederick notices the change on-camera. Also, the "cat hit with a dart" sound-effect was Mel Brooks's on the spot improvisation.
Another improvised moment by Marty Feldman was when Madeline Khan's character arrived at the castle. After saying the line "Soitenly. You take the blonde, I'll take the one in the toiben," Feldman starting biting and tearing at Khan's fur piece, which was not expected from neither the other actors nor the crew. This choice was eventually kept in the final cut, but multiple takes had to be shot because both actors and crew couldn't stop laughing at that point in the scene.
In The Young Lions, when Marlon Brando's character is fatally shot, he falls down a big hill and into a pond. He apparently injured himself rather badly in the fall, but being the world's most famous Method actor, he kept still and finished the take and waited to yell in pain until "cut" was called.
In You've Got Mail, there's a scene where the Tom Hanks character, holding balloons in one hand and a bagged goldfish in another, accidentally closes the door on the balloon strings. In an ad-lib, Hanks re-opened the door to free the balloons and joked to Ryan, "Good thing it wasn't the fish!"; it made the cut.
During the graveyard scene in Zoolander, right after Prewitt explains why male models are trained to be assassins, Ben Stiller completely forgot his line, and tried to wordlessly re-start the take by repeating his earlier line of "But why male models?", which prompted David Duchovny to run with it and hilariously reply, "...You serious? I just told you, like a minute ago."