In the Mash episode "Abyssinia, Henry," the final page of the script, in which Radar comes into the operating room and announces that Col. Blake's plane was shot down with no survivors, was handed to the cast a few minutes before the scene began. The scene in question was so shocking, an urban legend sprang up that the cast didn't know about the death until Gary Burghoff read his lines on the air. What really happened was that, with the exception of the director, none of the crew knew about the death, and their gasps of shock upon hearing the line ruined the first take.
The Brady Bunch: In the episode "Bobby's Hero," where Bobby took up outlaw Jesse James as a role model, the episode ends with a dream sequence where Jesse James shows up and shoots Bobby's entire family (even Alice!) to death (in an extremely silly-looking way, of course). To counteract the silly action, Lloyd Schwartz took actor Mike Lookinland alone to a closed set and began to describe to him how the scene would look in graphic, horrid detail, using Lookinland's real-life family as an example. The looks of terror you see in Bobby's eyes are from Schwartz screaming at him about how his real-life parents and siblings (even his pets!) were screaming in pain, suffering, bleeding, and dying. Schwartz, in his memoir about the series, says he and his father, Sherwood, were proud of how the episode came off as a non-preachy "anti-gun" episode.
The famous Sesame Street scene announcing the death of Mr. Hooper subverts this trope. Everyone in the cast loved actor Will Lee, so they all fought back genuine tears. They barely got through it. One line didn't come out quite right, so a second take was attempted. The cast didn't make it through the second try. As a result, the first take was used, in an unusual application of Throw It In.
In the Supernatural episode "In The Beginning," Samuel is being possessed by the YED and after Dean unwittingly tells him everything, he pins Dean to the wall and smells his neck while asking him if he's one of the "special children." According to Mitch Pileggi, the neck-smelling wasn't rehearsed beforehand so Dean's squicked-out reaction is completely genuine and the sexual subtext is even more creepy.
Another example is Misha Collins' first appearance as Castiel. The staring and ignoring Dean's personal space was not scripted, so that Jensen Ackles was genuinely surprised.
In Mad Men Season 3 episode titled "Guy Walks into an Advertising Agency" when Mr. Ford's foot is run over by the lawnmower blood is sprayed into the face of several onlookers. The director told them they would be sprayed on the count of 3, but instead went on 2. The shocked looks on their faces is a real reaction.
Sort of subverted in the Stargate SG-1 episode "Urgo", where Dom De Luise ad-libbed a lot of his lines and unintentionally made it difficult for Chris Judge not to break his characters stoicism. As a result, he has fewer scenes than usual in this episode.
Subverted in another scene, where Carter surprises O'Neill by humming to herself. Amanda Tapping had originally wanted to surprise her costar by humming the MacGyver theme. Unfortunately, no one on set could remember what it was.
In the Red Dwarf episode "D.N.A.", Lister is handed a photo of a man's genitals and reacts accordingly. During rehearsal, the photo was always something mundane, but when they actually shot the scene, Craig was given a photo of a guy's crotch without warning.
Another scene has the cast pushed against a metal grate with freezing cold water pouring over them, their screams at this point were genuine.
According to Patrick J. Adams (Mike), in Suits there's an element of this for scenes where Meghan Markle's character, Rachel, is jealous of Mike's other love interests. He used to talk up the actresses playing them to Markle between scenes in order to make her jealousy more real.
In I'm Alan Partridge, every character looks genuinely shocked when they see the contents of Alan's drawer, suggesting this trope.
In the Battlestar Galactica episode "Act of Contrition", when Starbuck tells Commander Adama she's responsible for the death of his son, Zak, Edward James Olmos scared actress Katee Sackhoff into thinking he was actually going to hit her, which is why she puts her hands over her head as she walks out of his cabin.
Olmos enjoys doing this sort of thing. The kiss in "Resurrection Ship part II" was also unscripted, as was the business with the wedding ring in "The Hub". Good thing Mary McDonnell is used to him.
He also stayed up for the correct length of time prior to the filming of "33".
During the pilot of Firefly, Mal and Jayne throw a body out the ship's airlock and rush back inside as the door closes with a fraction of an inch to spare. This isn't just feigned: Nathan Fillion and Adam Baldwin had no idea that Joss Whedon started closing the doors the moment they went out, to simulate how fast these characters had to act in their escape from the world.
In 24, Kiefer Sutherland changed the line of the famous "Jack whispering to Nina" scene from Day 2 from its scripted one to a declaration of love for Sarah Clarke in order to get a shocked reaction from her.
In the episode "Waking Moments" of Star Trek: Voyager, Tuvok dreams that he reports to the bridge naked. The people who are already there burst out laughing when they see him - and it's not acting. Apparently, Tim Russ attached really big fake genitals over his own, just to get the right reaction.
Geordi LaForge, blind engineer on Star Trek: The Next Generation, wears a metal sensor package called a VISOR over his eyes to permit him to see, but the stream of sensor data tends to overwhelm his brain and give him headaches. Actor LeVar Burton had a similar problemthe bolts used to keep the VISOR prop secured firmly against his temples were so tight that, twenty minutes into a day of shooting, he would start getting headaches.
Star Trek: Deep Space Nine: Garak suffers from acute claustrophobia. Although it had been hinted at a couple of years before, the episode where it's finally revealed that he genuinely does suffer from it occurs as a result of the character being locked in a tight enclosed space to rewire some communication panels to save everyone's life. This wasn't just a problem for the character, it was a problem for the actor, as the reason Garak was given acute claustrophobia was because his actor suffers from it in real life.
In "Falling Toward Apotheosis", one of the reasons that actor Peter Jurasik looks as he does when talking to the severed heads is that one of the heads was based on fellow actor Andreas Katsulas. They avoided telling him about it in advance.
In the episode "In the Shadow of Z'Ha'Dum", Andrea Thompson really slapped Bruce Boxleitner, very hard. Not only his reaction but the sound the slap makes is real.
On top of that, in the commentary, JMS says that she KEPT doing it in take after take. And they ended up using the first one.
During the news broadcast in "Severed Dreams", the startled reactions of the newscasters are genuine; when part of the ceiling fell in, it landed closer to the actors than intended.
In "Partners in Crime" Donna's mime scene was written in the script with only the words/phrases Donna was trying to convey, forcing Catherine Tate to make up the gestures on the spot.
In "Silence In The Library", new character River Song is introduced. David Tennant's confusion during his scenes with her is absolutely genuine, because Steven Moffat refused to tell anyone where he was going with the character.
In Mork and Mindy. Much of Mork's dialogue and antics were ad-libbed by Robin Williams, and so Mindy's surprise and confusion were often genuine. In one "It's A Wonderful Life" episode Mindy isn't supposed to react to the invisible Mork's antics, but Pam Dawber is visibly struggling to keep a straight face.
On Top Gear, the basic ideas for challenges often come from the presenters themselves, but the details come from "the producers". Clarkson, Hammond, and May are frequently pleasantly (or not-so-pleasantly) surprised on-camera by the contents of the infamous gold envelope telling them what they need to do next.
This clip from The Daily Show, in which John Oliver reads out a list of funny names. Between rehearsal and the final recording, the list was changed. Nobody told Jon Stewart.
And when Wyatt Cenac changed his metaphor for exactly how dry his martini was between rehearsal and the final cut. (At 7:35 if you don't feel like watching the whole thing.)
There's an episode of I Dream of Jeannie where Jeannie is trapped inside a champagne bottle. Filming for it involved an oversized mock-up of the lower part of the bottle, which Barbara Eden couldn't get out of without help. In order to get a realistic performance from her for the scene where Jeannie bangs on the glass and cries for help, the director had everyone on the set leave for lunch and pretend they had forgotten Eden was in the bottle, while a camera was actually still rolling. The result is in the final cut of the episode.
The Price Is Right occasionally does this with a showcase (usually April Fools' Day showcases) so that the model(s) involved are genuinely surprised. For the "Janice Pennington, This Is Your Strife" showcase, the cast and crew even went to the trouble of rehearsing a fake showcase with Janice.
The fifth series of the UK version of The Apprentice had the contestants create an advertising brand for a new cereal, including a mascot and TV commercial. One group came up with a superhero character called "Pantsman" who wears his underwear over his outer clothes, and made an advert featuring two young children, with "Pantsman" told to hide before filming so the kids wouldn't see him. Their expressions in the finished ad (as he walked in and they saw him for the first time) are priceless.
Frequently in Scrubs when the Janitor does a freakish rant, the astonishment of the characters listening is often genuine. Neil Flynn was given a long leash with ad-libbing lines, with the script often literally stating "Janitor: What Neil Says".
Specific example: One teaser has Elliot telling old jokes and having everyone else finish them for her. She finally shrieks "STOP FINISHING MY AWESOME JOKES!" Sarah Chalke didn't tell anyone she was going to play the line like that, and J.D.'s holding his ear and yelling "Oh my God!" was a real reaction on Zach Braff's part.
In Monty Python's Flying Circus, the shot of the kangaroo-hopping Freemasons with their trousers down and spotted boxer shorts displayed (part of the "How to recognise a Freemason" sequence) was filmed on a real London street with the then largely unknown Pythons dressed up in their banking suits and blending in. At a prearranged signal, they dropped their trousers and started hopping, and the shot was taken by a camera in a passing vehicle. The reactions from the passers-by are all completely genuine.
The scripts for Curb Your Enthusiasm are just outlines directing the flow of the conversation, and the actors are only allowed to read their own scenes. According to Larry David, Richard Lewis's knowledge of his scenes is even more restricted than this, because once David heard him use a line that he knew Lewis had planned.
In the pilot episode of LOST, Jack, Kate, and Charlie come across the plane's cockpit angled upright in the jungle. The cast members had not yet seen the set before filming began, so the looks of wonder on their faces were legitimate.
Also, L. Scott Caldwell and Sam Anderson intentionally did not meet until the filming of Rose and Bernard's reunion in "Collision". It really says a lot about the actors when you consider how heartwarming the scene is.
In series 3 of Skins, there's a scene where magician JJ breathes fire. Kaya Scodelario, Luke Pasqualino and Jack O'Connell (Effy, Freddie and Cook) were all told that the fire would be added in post-production as a special effect - nobody told them that they'd actually taught Ollie Barbieri how to perform the stunt. Kaya's scream is completely genuine.
And there's a failed example in series 4, in the scene where Naomi and Emily discover Sophia's shrine to Naomi in her army cadet locker. The plan was to prevent Lily Loveless and Kathryn Prescott seeing the shrine until it was opened during filming, so that they could play the characters as weirded out by The Reveal as possible; unfortunately, Lily ended up having a monumental job to stop herself laughing at the sheer crazy of what they found.
In the Hogfather TV film, when Michelle Dockery has to ride the hogs at the end, the director Jean Vadim kept her working for hours, finding picky fault after picky fault with her performance until she was literally screaming with anger, exhaustion and frustration. That was the shot he wanted.
An episode of Taxi called for Louie DePalma to whisper something to Elaine Nardo, and for her to respond by slapping him and saying "That's disgusting!". Danny DeVito whispered such sweet things to Marilu Henner during rehearsals that she was genuinely shocked when he started whispering not-so-nice things, and she blew several takes because of it (to DeVito's delight).
When Carol Kane joined the cast as Latka's girlfriend Simka, Andy Kaufman taught her their country's "language" by inviting her to dinner and refusing to speak English or let her do so.
In one episode, guest star and real-life boxer Carlos Palomino accidentally delivered a real left hook to Tony's face. You can see Palomino pull his hand towards his mouth in horror for a second on realizing what he did before getting back in character and turning around to exit.
The episode "Revelations" of Criminal Minds called for the UnSub of the week, played by James Van Der Beek, to pick up Reid by the shirt and verbally terrorize him. The threats that made it onto the screen were comparatively tame; however, in the DVD commentary on that episode, actor Matthew Gray Gubler reveals that the episode's director instructed Van Der Beek to shout whatever frightening obscenities he could think of in order to provoke a realistic, terrified reaction shot. The result was apparently so impressive that the writers were "pretty sure that he was actually going to kill him," and so filthy that Gubler couldn't repeat them, even in the DVD commentary. And it shows in the reaction shots.
Armando Iannucci likes to enforce method acting while directing The Thick of It. The scripts are often changed without the knowledge of certain actors to make their reactions more convincing. In addition to this the show is partly improvised, so the actors constantly have to come up with new lines- and as the show is a comedy, they have to be funny lines. This pressure makes everyone look as panicky as... well, the incompetent staff of a busy government department.
Playing Malcolm Tucker means Peter Capaldi has the most lines to learn, and he often stays up all night learning them only to arrive on set the next morning to find they've all been rewritten. The resulting stress and sleep deprivation help to make him look the part- off-screen he looks about ten years younger.
In one improvised scene which was never used, Glenn Cullen broke Julius Nicholson's glasses. Actor Alex MacQueen, who plays Julius, was convinced that James Smith, who plays Glenn, had broken his own prescription glasses, and not the pair Smith had discreetly switched them for. MacQueen was apparently so upset that the scene couldn't be used.
For the enquiry episode in season four, the cast were given no rehearsal or preparation before filming. Furthermore, there were no opportunities for the actors to talk to one another between reshoots, and those "testifying" were kept separate from the "panel", like an actual enquiry.
Blake's 7 is full of unexpected explosions: the reactions (shrieking and/or being thrown through the air) were often genuine, because the directors neglected to warn the actors about just what was going to go off and where.
Of particular note is the final scene of "Bad News". Jason Segal was not told what the titular "news" was. Alyson Hannigan's line of "Your father had a heart attack. He didn't make it." prompts a brilliant reaction from Segal, who causes the entire audience to cry at his voice breaking.
The scene in Breaking Bad where Walt comes into the kitchen after shaving his head was actually the first time Anna Gunn and RJ Mitte had seen Bryan Cranston with his shaved head; Gunn had specifically avoided meeting with him until then to help her reaction.
In an episode of The Adventures of Superman called "Night of Terror," Lois Lane is supposed to be knocked out when a thug punches her. Unfortunately, the actor accidentally missed his "air punch" and really did hit her, knocking Phyllis Coates unconscious. She had to go to the hospital, and he felt incredibly guilty.
In the episode of Home and Away with Sally's first wedding, Gypsy stands up and interrupts Kieran's vows to reveal that he had been hitting on her since he arrived. While filming this scene, Kimberly Cooper hit her legs on the pew in front of her, meaning that the tears in her eyes were "reeeeeal pain tears."
Occasionally happens with the actors on the hidden camera show What Would You Do?. For example, in one episode, a pregnant teenager denies her unborn child to some expectant adoptive parents.note All an act, of course, to see how people would react. During the scenario, two ladies approach the actress playing the sobbing mom-to-be. The women comfort her, and one of the ladies says a prayer about motherhood, moving the actress—WWYD veteran Traci Hovel, who had been fake-crying up to that point—to genuine tears.
Two notable cases happened in the Hollywood Director where Colin Mochrie would pretend to be an overly-picky Hollywood director, and always angrily yell "CUT CUT CUT CUT!". In one occasion, Chip Esten jumps onto Ryan's back and Colin actually means it when he says "CUT CUT CUT CUT!" because Ryan has a bad back. In another one, he asks the performers to do the scene backward and when he comes on, he's surprised they actually did it.
In Band of Brothers, the cast members didn't see the concentration camp until they were actually filming because they wanted the expressions of shock and horror to be as genuine as possible.
Cory Monteith and Dianna Agron were not allowed to watch rehearsals for "Lean On Me'; the reactions that we see during the performance were the actors' reactions on seeing it for the first time.
Dick & Dom in da Bungalow was unscripted anyway, but the producers liked to try and surprise the presenters- for example, by having somebody unexpectedly burst in through the door. That is, actually throughthe door. The hosts manage to turn genuine surprise into Played for Laughs collapsing in shock. (On another occasion, however, they are reduced to silence for a good thirty seconds simply by a lovely girl turning up when they expected a nice motherly cleaning lady, in a game of Make Dick Sick.)
In the Season 5 episode "Real Me" of Buffy the Vampire Slayer, director Joss Whedon made faces behind the camera to help Sarah Michelle Gellar's laughter seem more genuine when she learns of Harmony having minions. (This is at least partly because Gellar, at least at the time, was somewhat notorious on set for being unable to laugh on cue—one scene in season 6 where they come back to a scene where she and Giles are supposed to be on the tail end of laughing themselves sick has her facing away from the camera, bent over a pommel horse.)
Carnivāle opened with the premise that one of the two superpowered people in the main cast was destined to be revealed as a "Creature of Light", while the other was destined to be revealed as a "Creature of Darkness". To heighten the moral ambiguity behind Clancy Brown's character Brother Justin Crowe, the Creature of Darkness, the show's creator initially kept Brown in the dark about which of the two his character was supposed to be. Though Brown eventually caught on, it made his performance as a Knight Templar priest very believable in the earliest episodes, as he genuinely believed that he might be playing the show's hero.
Rather sickeningly occurred in an episode of Jim'll Fix It, where a child whose wish was to appear in kids' school drama Grange Hill played a scripted scene, which was then followed by an unscripted and unexpected scene where he was subjected to school bullying. Even after he realised it was a set-up, the child did not look happy at all.