Alice is a famous actress working on her latest movie. The time has come to film the scene where she rides a motorcycle through flaming hoops over a shark tank. But, wait, Alice is far too delicate, precious and non-expendable to put be put through that! A stunt double is needed. But Alice's usual stunt double is in the hospital, so what do they do? Hire Alice's friend Bob. Sure, he looks nothing like her, but it's not like anyone will notice.
A subtrope of Paper-Thin Disguise or Special Effects Failure, depending on whether the example is in-universe or not. It often overlaps with Fight Scene Failure. The Other Darrin is something similar, where a character gets a new actor because the previous one left the show.
Wet Hot American Summer: Whenever Neil climbs on the motorcycle it not so seamlessly cuts to another man who looks nothing like him wearing the same costume, in a cheap wig riding away. At one point the double even slowly and awkwardly rides the motorcycle directly toward the camera. After panning away to others, it whips back to show the actor now straddling the bike.
An infamous scene from Men In Black reveals that Will Smith's stunt double during the "Remember the little red button? Push the little red button" gag was Caucasian.
Averted in the climbing scene at the beginning of Mission: Impossible 2. Even in the interviews in the DVD extras, Tom Cruise's climbing double looks exactly like him.
This is a Bollywood movie called Yaarana. There are two main protagonists, Bishan and Kishan. Bishan buys gliders for both of them, and they take off. It's very obvious that the stunt doubles riding the gliders aren't the two actors.
When Jason grabs Chris as she is falling from a window in Friday the 13th Part III, it becomes obvious from the camera shots outside the house that a stunt actress is being used.
Big Bad Prof. Glastenbury alternates between his actor and a stuntman in a bad wig during the final fight in Avenging Force.
In The Great Muppet Caper as Ms. Piggy is racing to the gallery on a motorcycle there are several shots of a full body driver on the bike which is obviously a real person. Though they are careful to never show the face, the difference in size between a human and a Muppet is blatantly obvious. (and this is the same film that had some of the most elaborate actual Muppets riding bicycle scenes).
Batman: The Movie: In the various fight scenes the Riddler looks nothing like Frank Gorshin, who played him the rest of the time.
Diamonds Are Forever. In the scene where Willard Whyte's two female captors dive into the pool after James Bond, Thumper is clearly not the same woman as the actress who was playing her earlier.
Near the start of The Avengers, the Black Widow is being awesome. Only, Scarlett Johansson isn't. It's a really buff dude in a ginger wig. Usually, you'd be too enthralled by the fact they're flipping around strapped to a chair, but the lack of boobs, change in hair color and the actress' weight in additional muscle makes it a bit obvious.
While the shot's convincing in the movie proper, the outakes in the credits of Jesus Christ Vampire Hunter show the double for Mary Magnum's motorbike stunts without his helmet on (but wearing the, presumably padded, catsuit).
Salem from Sabrina the Teenage Witch: it is fairly easy to tell the difference between when he is being portrayed by a real cat and when he is being portrayed by a puppet.
Knight Rider had lots of scenes where David Hasslehoff's stunt double was used, mostly driving scenes but also some brawls. The stunt double's head often had a comically big silhouette due to an afro-like hairstyle.
The most infamous example from Star Trek: The Original Series might be the fight in "Amok Time", which featured a stunt double that looked nothing like William Shatner fighting an equally non-Leonard-Nimoyish stuntman.
Though you could also cite the fight between Ricardo Montalban's stuntman and whoever was doubling for Shatner in "Space Seed".
Or the fight in "Court Martial", where seemingly two random guys fought in place of actors William Shatner and Richard Webb.
Buffy's stunt double was quite a bit larger than she. During one commentary, Joss Whedon says of the switch that Buffy "straps on her fightin' boobs". Perhaps the most egregious, however, was when a heavyset cafeteria lady was replaced by a stunt double half her size.
This is why female rangers in Super Sentai/Power Rangers have skirts on their outfits, to hide the the fact that most of the in-costume work is done by stuntmen.
When some of the characters from M*A*S*H are riding in a chopper, and it's clear it's not the actual actors.
Wonder Woman: Lynda Carter's doubles were all not-particularly-effeminate men.
The first episode of Chuck has Chuck being burglarized by a "mysterious" masked ninja, later clearly revealed to be Sarah. The masked figure, however is clearly a powerfully built male.
In-universe and parody examples:
Anime and Manga
From Binbougami Ga, this is actually what Nadeshiko's "body doubles technique" is. A bunch of other people dressed in her outfit and a wig.
Played for laughs in I'm Gonna Git You Sucka. When Jack Spade's mother fights Mr. Big's toughs in the diner, she's replaced by a blatantly obvious stunt double - a guy in a wig, with a mustache. You can watch the scene here.
Invoked in Scott Pilgrim vs. The World when Scott has to battle Lucas Lee's stunt doubles, some of which are roughly one head shorter than Lucas Lee himself and a few of which appear to be of different ethnicities. Note that the Lucas Lee's stunt doubles are played by Chris Evans' actual stunt doubles.
Saturday Night Live: Kristen Wiig and Mike And Molly's Melissa McCarthy do a jazzy dance behind a sheet, so we only see their outlines. McCarthy's is about 150 lbs thinner than she is.
From an episode of VH-1's I Love Toys, the commentators were talking about pogo sticks; at one point, comedian Patrice O'Neal wants to try one out, but knowing how large he is, he wouldn't have been able to pull it off, and needed a stunt double. Cue a super skinny guy (wearing Patrice's outfit, which just hangs off his body) bouncing on the pogo stick.
World's Dumbest...: In an episode of "World's Dumbest Partiers", John Enos and Loni Love watch "their" wedding video (a couple's wedding video with their faces plastered over the bride and groom) involving an accident with the bride and groom crashing into each other on a zipline. Loni comments, "Good thing I used that white woman as my stunt double!"
In the second Broken Sword game, George wanders onto the set of a movie where the actors' stunt double is significantly fatter than the star he's doubling for. When said double is injured George serves as one as well.
In The Movies: Stunts and Effects your films are penalized by critics if you use a stunt double who is of a different race or gender than the actor they are representing.
The central joke of The B-Movie Comic is that it's recreating B Movies in all their cheesy, low-budget glory. Part of this is the variety of behind-the-scenes bonus comics, explaining how they accomplished the numerous stunts and special effects. Turns out that Snuki's actor does his own stunts, and everyone else's stunts, regardless of how little he looks like them.
SfDebris highlights this in his reviews when it pops up, but also explains why it does with older shows. Basically on old, small, fuzzy tv sets of yesteryear the technology meant that even the most superficial resemblance could be gotten away, but as technology advanced and we got better and bigger televisions it became more noticeable when a non-lookalike stunt "double" was used.
In Stewie Griffin: The Untold Story, there's a scene of Stewie leaping out of a hotel window. It cuts to what appears to be Dennis Franz in red overalls making the dive.
Subverted in Kick Buttowski when Kick takes Scarlett's place as Teena's stunt double. It's fixed in editing so Kick looks exactly like Teena. Later, when Kick winds up in hospital, he's replaced with Emo Kid.
In SpongeBob SquarePants, it's obvious Sandy's doubling for Mermaid Man in the unofficial movie they make.
Used in the Looney Tunes short "A Star is Bored", where Daffy is Bugs' stunt double for any dangerous scene. He's dressed in a rabbit outfit but you can still see his duck face.
In the episode "The Return of Batduck", Plucky becomes the stunt double for Michael Keaton when Batman Returns is filmed. He's even referred to as the "Stunt Duck" by the Director.
In the episode "Kon Ducki", during the "Making of" segment, Plucky's original plan was to have Hamton as his stunt double during the scene where the mast falls on top of him. However, Hamton's inability to remember his lines led to Plucky having do his own stunts.
In The Fairly Oddparents episode, "Lights, Camera, Adam!", Timmy becomes the stunt Cleft for The Crimson Chin Movie. The Main difference between him and the main Actor playing Cleft is that the main Actor playing Cleft is a much taller teenager and Timmy is a short 10-year-old.
The Legend of Tarzan has an episode in which a director is filming a movie about a wild man raised by monkeys in the very jungle the protagonists live in. Since the actor playing the wild man is lousy at filming the action scenes, Tarzan ends up roped into being his stunt double. Not only do Tarzan and the actor look noting alike, but their outfits are different.
In the pilot for Sheep in the Big City, sheep finds himself cornered on a rooftop by Far Mer John and General Specific, though while Far Mer John and General Specific argue over the General's penmanship (or lack thereof) on the cue cards, Sheep uses this opportunity to escape: he unravels his wool, creating a rope for him to swing over to the roof of the building across the street. Cut to a shot of a man wearing a Sheep costume swinging from a rope to the other building, only crash against the facade, and tumbled down to the street below (all in slow motion, and with added film grain), before cutting back to Sheep, who is now lying on the cracked sidewalk in a daze.
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