Early in his television career, Edward James Olmos would ad-lib and insist on playing his scenes more naturalistically—which TV actors hate, because it takes control away from them in a scene. This outraged his co-stars, but the tension on set meant that he got his desired result anyway. Mind you, Olmos did this in his very first television role, before he had any clout.
Olmos was given a tape of Miami Vice before taking on the role of Lt. Castillo. He noticed how Crockett and Tubbs would smirk and trade looks when they were grilling suspects. Olmos came prepared. In addition to requesting a plain black suit and tie (which clashed wildly with the designer suits on display), he would face the wall when consulting with his detectives. When the camera cut to a reaction shot, Crockett and Tubbs weren't smiling—they were uneasy, confused. "They could not dismiss me."
In the M*A*S*H 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.
In the House, M.D. episode You Don't Want To Know, the patient of the week is played by Steve Valentine, an actor who is also a professional magician who is equally good at both stage and close-up magic. In the scenes where his character performs close-up magic while the doctors are trying to diagnose him, Valentine actually performed the genuine close-up magic-tricks. The regular cast were not warned ahead of time. Their surprised looks are genuine.
The famous Sesame Street scene announcing the death of Mr. Hooper. 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!.
Before then, there was a scene where Big Bird had to accept that Snuffy wasn't real, and they both started to cry over it until Big Bird realized that his feathers were wet from Snuffy's tears, so he had to be real. After filming the scene, Carroll Spinney and Jerry Nelson removed their masks to reveal that they were actually crying.
A lot of segments with Jim Henson and Frank Oz (particularly as Ernie and Bert) are loosely scripted and are largely the two actors playing off each other. One segment ends with the painting in the duo's apartment falling off the wall, but it fits so well as a punchline that the camera keeps rolling (and Bert, not missing a beat, screams with surprise).
Sing and Dance with Barney provides an odd case. Apparently, no one in the cast was told about the Confetti Drop at the very end. A bunch of the kids jump; the oldest among them seems to spend the last few seconds trying to figure out the ceiling mechanics of the drop.
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.
A third example is during filming of the season three finale "No Rest for the Wicked." The pain of Dean being strung up by chains in hell was real for Jensen Ackles. To film the scene, Jensen Ackles was lifted 13 feet in the air by wires and a waist harness. Unfortunately, the waist harness slipped, causing its buckle to continuously dig into his hip throughout the scene's three or four takes. The actor, who "had tears rolling down [his] face" as he was lowered down, deemed it the most physical pain he has endured for a single shot. 
The episode "A Very Supernatural Christmas" features Dean's surprised reaction at the strength of the eggnog Sam's just handed him. Jared Padalecki spiked the glass with real booze—a lot of it, according to Ackles.
Season 1's "Bugs" resulted in real bees (thousands) to be used in several shots for several takes. Needless to say, the actors were stung multiple times and then found out later that the bees didn't show up well on camera and had to be CGI'd in.
Jensen dove body first into real glass in the premiere episode of Season 4, resulting in multiple real cuts (You can see a bandage in the take where he starts to get up).
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 episode "Urgo", where Dom DeLuise ad-libbed a lot of his lines and unintentionally made it difficult for Christopher Judge not to break his character's 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. Instead, she hums the SG-1 theme tune.
In the Red Dwarf Series 4 episode "D.N.A.", Lister is handed a photo of Kryten's genitals and reacts accordingly. During rehearsal, the photo was always something mundane, but when they actually shot the scene, Craig Charles was given a photo of a guy's crotch without warning. Or something, the story varies.
Series 7 episode "Duct Soup" has a scene where the cast pushed against a metal grate with freezing cold water pouring over them; the water really was ice cold and their screams of shock were genuine.
In Series 10's "Father and Suns" an ill Lister delivers an anguished rant while heavily sweating and looking like he's about to throw up. Craig Charles was genuinely very ill.
A lot of Rimmer and Lister's snide remarks and mockery to each other in early series can seem quite genuine, that's because for a long time, Chris Barrie and Craig Charles genuinely hated each other.
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.
In "Relics", Scotty heads to the holodeck, ordering a recreation of the original Enterprise bridge from Star Trek: The Original Series. Scotty's look of amazement was James Doohan's real reaction, since this was the first time he was seeing this set.
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.
The First Doctor's increasingly irascible and contemptuous demeanour was said by Peter Purves, who played the companion Steven towards the end of William Hartnell's tenure as the Doctor, to be partially a result of the memory problems that Hartnell was experiencing. His inability to remember his lines and the extra time and effort required to get them right would make him flustered, exhausted and annoyed with himself, which made him play the character with a sense of Inferiority Superiority Complex, weary-old-man pathos and pent-up frustration. Purves said he found it fascinating.
Similarly, in many later scenes, you can see Hartnell's eyes flicking about in deep thought as he concentrates really hard on remembering his lines - which again works, as his character is an Impossible Genius with a lot of mysteries and secrets, and it makes him appear to be constantly calculating his next moves.
At one point, Spandrell has to read aloud a disturbing letter that the Doctor has written for him. In rehearsals, he used a note written in English and thus had not bothered to learn the lines. When it came time to shoot the scene, the director and set designer switched it for a note written in "Gallifreyan" without telling the actor. The second Spandrell opens the note, he goes visibly pale and sweat springs from his forehead, and stumbles over his words as he struggles to remember them - the appropriate shocked reaction.
There's a part where the Doctor is sitting on a chair trying to manipulate Spandrell. Spandrell begins explaining why he can't help the Doctor, and the Doctor shoots up from his chair mid-line and looms over Spandrell to intimidate him with his size - he audibly falters and his voice shakes. This was an adlib from Tom Baker and the actor playing Spandrell was just reacting with surprise to Tom unexpectedly going off-script.
The infamously brutal cliffhanger in "The Deadly Assassin" in which the Fourth Doctor is held underwater by someone attempting to drown him ended up being a bit more horrifying than anticipated thanks to the genuine terror in Tom Baker's eyes, as he cannot swim and has a terrible phobia of bodies of water. The director praised his intense performance, but both of them expressed a lot of worry that it was too frightening for the show - concern shared by deranged Media Watchdog Mary Whitehouse. The ongoing and messy media furore created by how legitimately upsetting the scene is to watch was a significant factor in the show getting ReTooled into a Lighter and Softer format later on, with the Doctor's character being altered from someone who did experience occasional terror and pain, into a Trickster Archetype version of The Ace (bordering on Boring Invincible Hero) who didn't react to situations with as much emotional depth. (Tom Baker's water-phobia had previously affected his performance negatively in "The Android Invasion" which called for a Reed Snorkel sequence in a lake, followed by a comical scene of him emerging with his hair soaked down, spitting out water. He's doing his best to make it look funny but from the look in his eyes you can tell he's inwardly bricking it.)
In "Logopolis", after the Fifth Doctor sits up after having regenerated from the Fourth, Peter Davison interpreted the tone of the director telling him to do it as an indication that he'd done something wrong, and so broke character briefly. Fortunately, this works really well, because he looks at the camera with a look of nervous, surprised confusion which perfectly sums up how one would feel upon turning into an entirely different person after dying doing some reckless stunt.
Self-enforced method acting in "Castrovalva": In the last scene, the script called for Adric to look "pallid" as a result of his mistreatment by the Master. Matthew Waterhouse accidentally achieved this by over-indulging in Campari the night before, so that he was severely hungover for filming. At one point, he was actually being sick behind a tree off-camera.
Happened to Davison all over his final story ("The Caves of Androzani"), so much so that he joked in the commentary that he thought that they were trying to actually kill him off. The Doctor's reaction to getting backhanded by Sharaz Jek is completely genuine; the actor playing Sharaz Jek couldn't see well out of his mask, missed his mark and couldn't see Davison because of the restrictions of the mask and actually backhanded him. The part where the Doctor throws himself away from the explosion in the sand is because the sfx guy set off the explosion too soon and shot sand into Davison's eyes.
In Mork & 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" being 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 series 3 and 4, much of the "stage fighting" Kathryn Prescott does is real, and the person she's hit is in actual pain. Particularly notable in in the Series 3 Loveball episode, where she actually hits Doug (played by Giles Thomas). His surprise and pain in the scene is very real.
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.
The ending is a crossover of this and Wag the Director: Gareth Thomas was fed up with the show and really determined that Blake would be so unambiguously dead that there would be no way to bring him back if the show got renewed again. Therefore at every opportunity he went back to the special effects crew to ask them to put another Squib and bloodbag under his costume. As a result, when the shooting scene finally happened the blow to his chest was hard enough to really wind him and leave him cut and bruised.
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.
When Walt comes into the kitchen after shaving his head, that 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.
Cranston did it again in season 4. He made sure the writers didn't tell him whether or not Walt had poisoned Brock. Because of this, when Jesse accuses him of doing it while holding Walt at gunpoint, Walt's defense of himself sounds completely honest, highlighting how effective of a liar he can be.
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.
The series' next Lois Lane (Noel Neill) was also subjected to this in the episode "The Golden Vulture," which ends with Clark Kent clinging to the side of a boat and asking Lois to help him up. Noel wasn't warned before filming began that George Reeves would then pull her into the water. (Jack Larson confessed in an interview for the DVD extras that this was because Clark was supposed to do this to Jimmy Olsen, but he refused.)
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.
Brittany's famous non-sequiturs were originally adlibbed by Heather Morris. The other actors' confused expressions were real.
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.
The climactic scene of the classic Swedish TV version of Emil of Lönneberg, where Emil drives his sick best friend Alfred to the Doctor through a raging snow storm, was filmed by placing the actors in a freezing snow field in Northern Sweden and then pointing a very large wind cannon at them. According to Björn Gustafson (Alfred) they could only film for a minute at the time and had no problems pretending to be dying from cold and fatigue.
In an episode of Parks and Recreation, Andy Dwyer shows up at Leslie's house naked due to a miscommunication. Apparently in the first few takes, Amy Poehler's reaction on opening the door wasn't convincing enough, so Chris Pratt took off the skin-coloured briefs without telling her...
On a few episodes of Wheel of Fortune, contestants have solved their bonus puzzles on the buzzer, which of course requires a stop-tape while the judge reviews to see whether the answer was in time or not. In a few cases, the contestants are not told what the rulings are until the cameras start rolling again, leading to a more natural reaction to being declared a winner — as the home audience sees only the contestant solving on the buzzer, a near-seamless edit, then a natural reaction to the win.
In the episode "The Carpet" of The Office (US), Michael finds a very foul-smelling...thing...on the floor of his office. To gain the best reaction, Steve Carell threw a stink bomb into the set before shooting started in which everyone is trying to get a look at it. Depending on how late they started filming, the entire cast's reaction is real.
A rare non-human example in Prehistoric Park. In the arc of Martha the mammoth, the crew tries to see if she might possibly be accepted into an elephant herd so she wouldn't be so lonely. To join, Martha has to meet with the matriarch of the herd to see if the elephants will accept her. For filming this, the crew built a puppet of an enormous mammoth's head and used it to "converse" with the live elephant. They weren't sure what its reaction would be, but to their surprise the elephant was receptive to the puppet and greeted it like another living elephant.
In the fifth episode of Wolf Hall, Thomas Cromwell has to get through a Mobstacle Course in a tent to reach Henry VIII, who may be dead. The actors playing the crowd were supposed to make way for Mark Rylance to get through, but they were so caught up in the yelling and milling around that they were actually blocking him from Damian Lewis. The bits where he's grabbing them and shoving him out of his way are genuine.
In an SNL sketch from 1993, about the parents of two teenagers finding pot in the house, who then hire a motivational speaker (Chris Farley) to talk to them (and who "lives in a van down by the river"), the two young actors can't keep a straight face during his antics, and the boy's reaction when Farley jumps onto the table causing it to break is completely genuine. (The actors were not told that the table was a breakable prop, nor did they know that Farley was going to jump onto it.)
While they were told prior to filming the scene, The Reveal in season 1 of The Good Placethat they weren't in the Good Place at all was kept secret from most of the actors for much of the season, in order to avoid it influencing their performances.