Lost in Character

""I don't drop character 'til I've done the DVD commentary.""

- Kirk Lazarus, Tropic Thunder

This is when a character who is an actor (not an actor in Real Life who plays a character) goes so deep into their role that they end up temporarily forgetting their original self. They usually start Method Acting, and before anyone knows it are so immersed in the role they're playing that they almost literally become that role, forgetting their old name, life, and setting aside their original personality. It's worth noting that while this is analogous to Becoming the Mask, the character who becomes Lost in Character is not a criminal or The Mole, may not even like the role they have immersed themselves in, and has gone so deep into the role that they don't lament any Loss of Identity or even remember having been a different person (much less compare that life to the present).

While in this state the character may act against their own interests or those of their allies and loved ones, though a good slap may fix them, or pushing a Berserk Button their normal self would take issue with. If the character had a previous psychological disorder, this may result in temporary Loss of Identity or even forming one or more Split Personalities. In extreme cases it may take Deprograming to bring them back.

This trope can be considered a mundane, non-Phlebotinum version of getting hit with Laser-Guided Amnesia and having Fake Memories implanted. Contrast Brainwashed and Crazy and Split Personality Takeover. Compare with Becoming the Mask, where con men or The Mole who grow to like their assumed identity more than their original one, and also That Man Is Dead, in which the character emphatically rejects his old identity. Compare also Enforced Method Acting, where this is imposed on an actor.

Anime and Manga

 * In Yu-Gi-Oh Ze Xal, Fuya Okudaira, a child actor, is pressured by his Stage Mom to not speak with other children and to focus on his role as D.D. ES Per Robin. While Fuya wants to be friends with other kids, he doesn't want to disappoint his mother. Upon coming across a Numbers card (which amplifies the user's desires, usually to a sinister extent) that takes a form similar to his mother, he begins to believe that he really is D.D. ES Per Robin, and vows to protect his card, No. 83: Galaxy Queen, as he would his own mother.

Comic Books

 * Batman
 * Back when The Joker had his own comic series in the 1970s, one of the opponents he faced was an actor who had started to believe he actually was Sherlock Holmes.
 * The post-Zero Hour version of the second Two-Face, Paul Sloane, was reimagined as an actor so involved in method acting that he ended up turning himself into one of Batman's deformed, psychotic rogues while researching a part.
 * One of the possible deaths of Batman in Whatever Happened to The Caped Crusader has him trying, and failing, to talk down an actor who got lost in the supervillain part he was hired to play.
 * In recent interpretations, Spider-Man enemy the Chameleon sometimes has trouble discarding his assumed identities without some mental issues.
 * In a 1970s story, a combination of psychological conditioning and PTSD left Marvel Comics Super Spy the Black Widow mentally "stuck" in her cover identity as mousy schoolteacher Nancy Rushman.
 * This was part of Judge Doom's backstory in Roger Rabbit: The Resurrection of Doom, explaining why nobody knew who he really was until after the events of the movie.
 * The Vertigo reboot of Human Target made this a key part of Christopher Chance's success. He was so good at impersonating his clients that he actually built himself into their personalities in order to fool whoever was trying to take a swing at them, and required extensive deprogramming once a job was complete.

Film

 * This concept is a metaphorical interpretation of Black Swan. Nina, sweet and a perfect representation of the White Swan, tries so hard to become the Black Swan that she loses herself.
 * The 1947 Film Noir A Double Life is all about this, where an actor (Ronald Coleman) playing Othello on Broadway finds his life being taken over by the role and eventually follows in the part's footsteps when he murders his mistress.
 * Deconstructed and parodied in in Tropic Thunder by Robert Downey, Jr.'s character. And yes, that quote at the top of the page is serious too, because Robert Downey Jr. stays in character for the actual film's DVD commentary (Although this was an extended joke about the character he played).
 * According to the sequel comic, Judge Doom of Who Framed Roger Rabbit was originally a Toon actor named Baron Von Rotten, who often played villainous roles (a deleted scene of the movie claimed Doom was the one who shot Bambi's mom). A concussion during a shoot led him to think he really was a villain.

Literature

 * Kurt Vonnegut's "Who Am I This Time?", featuring an actor who entirely inhabits his roles, because he doesn't really have a personality outside of them. Later adapted to an excellent made-for-TV movie starring Christopher Walken and Susan Sarandon, and directed by Jonathan Demme.
 * In Robert A. Heinlein's Double Star, the main character is hired to impersonate a kidnapped politician. He becomes so immersed in being this man that

Live Action TV

 * In an episode of Buffy the Vampire Slayer Drusilla uses her psychic talents to appear to Giles as his former lover Jenny Calendar in order to seduce some information from him. She gets so carried away in the role that she continues making out with him for some time after learning what she was after.
 * For that matter, the actors themselves would do this on more than one occasion. Memorably, Sarah Michelle Gellar was so invested in her character's romance with Angel that when he turns evil and abuses her, in the scene where she runs home and cries she is so broken up they had to close the set for half an hour.
 * This was parodied in an episode of Community, when Abed told the story of creating and then becoming a character for his walk-on role in Cougar Town.
 * Danny Pudi filmed a cameo on Cougar Town in reference to this scene. It's glorious.
 * This is arguably the super-power of Echo, the protagonist of Dollhouse. Despite repeated memory wipes, she always retains the "imprints" the other personalities she's assumed.
 * Possibly Sophie Devereaux in Leverage. She mentions before her sabbatical that she's created so many fake personas she's not really sure what's really her anymore and leaves to bury each of them. This extends so far that we're not sure what her real name is.
 * This was the plot point in an episode of Monk when a TV movie was being made about the titular character. The actor had a history of falling too deeply into his roles and when he portrayed Monk... Afterward, the actor decided to try something less depressing, like Hamlet.
 * Sort of happened in real life with Jeremy Brett as Sherlock Holmes; he became so obsessed with the character that he only referred to him as "You-Know-Who", and later became ill and died.
 * Actually, Jeremy Brett became ill while obsessing over the character - realizing that he was manic depressive allowed him to start overcoming his obsession to the point where he could enjoy playing Holmes. He died eleven years after Sherlock Holmes first aired, and had quit the series entirely the year before his death.
 * In the Twilight Zone episode "A World of Difference" this is what everybody thinks happened to Gerry Raigan, an alcoholic actor who seems to have snapped and identified too closely with happy executive character Arthur Curtis. He, on the other hand, thinks that he actually is Curtis, trapped in a nightmare world.

Newspaper Comics
"General on TV: Holy bovines, Corporal! There's a giant monster invading the city!
 * This was also parodied in a 2009 Garfield strip, with the dialogue as follows;

Soldier on TV: That's not a monster, sir.

General: What are you talking about? Call out the artillery!

Soldier: It's just a bad actor in a rubber suit.

General: Oh, it is not! It's a monster!

Soldier: Come on... I can see the zipper.

General: Egad! A zipper monster! That's the worst kind!

Soldier: And that's not a real city.

General: Insolence! I'll have you court-martialed!!

Soldier: These are just tiny little model buildings.

Garfield: General Cordwood seems to have buried himself in the part.

Soldier: See?

General: PUT MY HOUSE DOWN!!"

Theater

 * Hamlet is sometimes interpreted this way, with Hamlet's feigned insanity leading to him actually losing his grip on reality.

Video Games

 * As with so many other tropes, this one is deconstructed in Metal Gear Solid: Decoy Octopus is so committed to his disguises that he requires deprogramming to leave his assumed identities after a mission.

Western Animation

 * In an episode Kim Possible The Fearless Ferret and White Stripe's actors forget they are actors and think they are there own characters.
 * In an episode of the "Super Chicken" segment of George of the Jungle, an actor portraying a whiplash-like character abducted the actress portraying the damsel in distress, making her a real Distressed Damsel.

Real Life

 * This is not unprecedented for practitioners of Method Acting, and can have very real and very negative effects on the actor in question. A good portion of acting training is in fact learning how to avoid this.
 * The Canadian actor Raymond Massey, best known for his many, many portrayals of Abraham Lincoln on stage, film, and television, eventually became so obsessed with perfecting his performance that he started to assume the vocal and physical mannerisms of Lincoln in day-to-day life, even appearing at social gatherings dressed in Lincoln-esque clothing. A friend said of his obsession, "Massey won't be satisfied until someone assassinates him."