Numbuh One: When I look into Lizzie's eyes, I can tell she's not a jerk.
The inversion of Something Only They Would Say: When a character is pretending to be someone else, they may unwittingly reveal this by saying something that would be out-of-character for who they're impersonating. Variations include not responding to a well-known Berserk Button, doing things they're normally afraid of (or have a similar excuse for never doing), expressing hatred for a nickname they insist to be called by, or otherwise invoking O.O.C. Is Serious Business. A common variation in Japanese media is using the wrong pronoun.
Often Invoked in kidnapping and I Have Your Wife scenarios, to let the heroes know that something is amiss. If the Big Bad is demanding a ransom, this is to alert them to the fact that it's a trap; if he wants the kidnapped to "assure" The Hero that the kidnapped is "in fact" okay, this is to secretly convey that they're not. Sometimes serves as a Quiet Cry for Help. If they've pre-arranged such an alert, this is a Covert Distress Code.
Real-life military personnel sometimes use hand signals when being taped to communicate in another way with their 'home base'. There are a few documented cases of soldiers giving hand signals (and one case of them just flipping the bird to the camera) to alert the people receiving it that no, they weren't being treated very politely at all. And some have done it just for fun, giving the sign for coercion when forced to shake a politician's hand, for instance.
Compare with Lying Finger Cross (a common gesture used in this trope), Not Himself, Ooh, Me Accent's Slipping, and Trust Password (a pre-arranged alert to confirm one's identity and freedom of action). Contrast with Something Only They Would Say (in which a character is identified by a characteristic) and Bluff the Impostor. Sister Trope to You Called Me "X"; It Must Be Serious and Batman Grabs a Gun. See also O.O.C. Is Serious Business.
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- Real Life
- In an early '70s TV commercial for the short-lived Cap'n Crunch variant Vanilly Crunch, the good captain's ship is approached by Wilma the Whale, the mascot for that cereal. After the captain does a description of the cereal, he shoots a cannon at the whale! Turns out that pirate Jean LaFoote (the "bad guy" in these ads) had disguised his now-sinking vessel as Wilma. Cap'n Crunch knew it because he described Vanilly Crunch and the whale didn't smile.
- In the first episode of The Elysium Project, Ian figures out that the Jessica that he's talking to is actually Mirage when "she" accidentally gives away that she doesn't know about the rendezvous point that they had set up with the other escaped subjects.
- In Big Finish Doctor Who play Intrusion Countermeasures: The Reesinger Process, Dr Allison Williams and Group-Captain Ian Gilmore are undercover in the eponymous interrogation training program, which is actually hypnotising people. When Gilmore gets sent out as a hypnotised assassin, the guy running the place (who has got the idea they're an item) tells Allison he's resting but he says he loves her. Allison is taken aback by this but decides it's probably military code for "everything's fine".
- In a nod to Star Trek, the Fighting Fantasy novel Starship Traveler has a part where you're captured by aliens and cloned, and the clone tries to impersonate you so the aliens can steal your ship. If you succeed on a roll, you can with great mental effort implant two things into the clone: an inability to lie, and the name of a certain sterile prison planet back in your own universe. When the clone tries to get your crew to beam down and get captured, he'll tell them that it's lovely down there, "as nice as [prison planet]".
- In Dungeons & Dragons third edition, the Sense Motive skill can be used to notice when someone familiar is under the effect of an enchantment. With subtle enchantments (which the subject may not even be aware of), the difficulty is rather high; however with Dominate effects, which drastically reduce the range of actions of the victim, the check is much easier.
- Fleuret Blanc provides an example of someone else setting up a person as out-of-character rather than acting out-of-character themselves: Junior tries to deflect blame for the anonymous text messages by pinning it on Le Neuvieme — but the messages are in English, and Le Neuvieme only speaks French. This allows you to figure out the real culprit.
- Phoenix Wright: Ace Attorney - Trials and Tribulations:
- Phoenix thinks this trope is in play in the first case when Dahlia starts acting all-around bitchy. After Mia revealed Dahlia to be a major Bitch in Sheep's Clothing, who only dated Phoenix to get a certain poison-filled necklace and tried to kill him when seduction failed, she didn't feel the need to pretend anymore. Phoenix, however, is still crazy in love with "Dollie" and, after the trial, claims that this Dahlia was a fake. In a way he's right. The girl he actually dated at the time was Dahlia's twin sister Iris, who really was a Nice Girl.
- When Iris starts acting like an Extreme Doormat and makes some Innocently Insensitive comments, it's a clue that Dahlia is impersonating her. Phoenix does notice that something's off, but since he met said person just two days earlier (or rather he thinks he did), he doesn't connect the dots immediately.
- In the Whateley Universe story "Test Tube Babies", team superboy Lancer is fighting a power mimic/shapeshifter who now looks like Lancer. One of them yells at a teammate with an anti-brick weapon to 'shoot both of us'. The teammate blasts that one senseless. Not only is the teammate savvy, but they have communicators they would really use instead.
- One episode of Atop the Fourth Wall has 90s Kid suggesting a Nirvana reunion that involves digging up Kurt Cobain's body and having zombie chicks play him like a puppet. It isn't until later, but it finally dawns on Linkara that not even 90s Kid is that twisted, leading him to realize he's been possessed by The Entity
- In To Boldly Flee, The Nostalgia Chick and Oancitizen infiltrate Zod's ship disguised as his old friends, Ursa and Non. Unfortunately, Non is mute, and Oancitizen can't resist talking. And he really can't resist singing...
- During the first episode of the Yellow Temperance mini-arc in Vaguely Recalling JoJo, Jotaro realizes that Kakyoin is a fake after witnessing Kakyoin in the act of devouring some beetles. Jotaro punches the fake Kakyoin in the face, which gets him to reveal himself as Rubber Soul. This is pretty much what happened in the arc it's parodying, oddly enough. It helped that Rubber Soul is a jackass who doesn't bother trying to mimic his target; he likes making them look like sudden assholes.
- In No Evil, Ichabod and Paula reach Kitty's house late at night and find that she appears to have stopped in the middle of canning some pumpkins and not cleaned up. Ichabod is dismissive of Kitty's "carelessness"; Paula instantly realises that there's no way Kitty would have left in the middle of a task of her own free will, and starts looking for evidence of whoever abducted her. Cue a late-night visit to Mictlan Wood, where Paula punches her way through rather a lot of undead.