A close relative of EvilTwin and FreakyFridayFlip, NotHimself is any plot where a character acts contrary to the way they normally behave. Can be used to provide a mystery to be solved, or to incite bewilderment in the other characters. The deviation is often a marked increase in [[DemonicPossession evil]], insanity, and/or horniness: the emergence of the [[ShadowArchetype inner shadow]] -- or a [[SpotTheImpostor switcheroo]].

Sometimes, it's a detective or police character behaving as if they were corrupt, or a criminal. This will usually turn out to be the FakeDefector.

If the change is deliberate to save their friends, it's an OutOfCharacterAlert.

If the change is caused by poor writing or is never given an in-canon reason, this could be CharacterDerailment.

Compare OutOfCharacterMoment. Contrast with SomethingOnlyTheyWouldSay. See also VirusVictimSymptoms.
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!!Examples

[[foldercontrol]]

[[folder:Anime & Manga]]
* Subverted in ''MagicalProjectS'' while Misao certainly wasn't acting like herself when she tortured the main cast as the [[DarkMagicalGirl evil magical girl]] Pixy Misa (she wasn't even aware of her other side) it was revealed that it was in fact herself (the repressed aspects of her personality),
* Mimi in ''PerfectBlue'' throughout the majority of the movie. Also, Rumi towards the end.
* Lelouch on occasion in ''Anime/CodeGeass''. As well as Suzaku and Rolo once he starts to become closer to Lelouch.
* Utena after having lost a duel in ''Anime/RevolutionaryGirlUtena''.
* Shinji of ''Anime/NeonGenesisEvangelion'' many times, as well as much of the other main characters, mainly Asuka.
* In the anime series ''Anime/TsukuyomiMoonPhase'', Hazuki has frequent periods of NotHimself, during which she "becomes" Miss Luna, a lustful, blood-thirsty vampire girl.
* Aside from [[MoreThanMindControl Ken]] during ''DigimonAdventure02'', the normally nice and affable Takeru has been known to become scarily angry when there is a threat to his Digimon and friends, as a result of witnessing the [[DeadSidekick death of his own Digimon]] in the first series. These angry moments have caused him to physically attack the then-Digimon Kaiser Ken ([[http://i75.photobucket.com/albums/i320/mpcp13/TakeruDarkAmusement.jpg while smiling in dark amusement at Ken's injury of his [Takeru's] face]]), advocate the killing of Dark Digimon, and deeply disturb [[FusionDance Jogress]] partner Iori on separate occasions.
** Just to put this into context, Iori is TheStoic of the series. To freak him out would seem to be a very hard task... unless Takeru's pissed.
* This usually sparks the plots in ''VisualNovel/HigurashiNoNakuKoroNi'', but one character stands out in particular: "The person who was there was not the Rika-chan I knew. It was another Furude Rika." Of course, the difference between Rika and the other examples is that [[spoiler:the "abnormal" personality is really her TrueSelf, her "normal" self is just ObfuscatingStupidity.]]
* In ''Manga/{{Bleach}}'', the 9th Espada, [[spoiler: attempting to trick Rukia by posing as her former mentor Kaien Shiba, betrays himself by making a request that was blatantly uncharacteristic of Kaien; suggesting that she could earn his forgiveness by killing all her friends. Rukia, enraged, says Kaien would never say that even as a joke. (Kaien also died apologizing to Rukia for causing her pain and thanking her for freeing his heart by performing a MercyKill on him).]]
* In ''DragonballGT'', a compromised Vegeta calls Goku "Goku" rather than "Kakarot", thus giving himself away.
** Goku and Gohan both qualify, as their [[SuperMode Super]] [[SuperpoweredEvilSide Saiyan]] transformations, at least initially, radically alters their personality from the incarnation of goodness and purity to cold blooded, driven-by-rage, homicidally sadistic {{Blood Knight}}s.
*** Basic corollary: The more pacifistic a Saiyan is in their base form, the more sadistic (and dangerous) they are as they increase in power. Vegeta and Gotenks barely see any change in personality in their Super Saiyan forms, while Gohan, who'd rather go to school and [[IJustWantToBeNormal be normal]], could give [[{{Naruto}} the KYUUBI]] a run for its money on bloodthirst when he's [=SSJ2=].
* Beware the InternetBackdraft when bringing this up with {{Mai-HiME}}, particularly with reference to [[spoiler:Shizuru Fujino]]. The anime is ambiguous about how much (if indeed at all) [[spoiler:the [=HiME=] star is affecting the [=HiME=] as it gets closer to earth. Is it playing with their emotions to provoke them to fight each other (thus causing Shizuru to go completely mad when she's rejected by Natsuki), or is it all a result of their normal psyches, just put under a lot of stress?]]
** Interestingly enough, [[spoiler:immediately after being resurrected, Shizuru [[TearsOfRemorse starts crying]] and [[MyGodWhatHaveIDone apologizes to Natsuki]], which she had not done even in the moments before her death, after Natsuki kissed her and told her that she loved her, albeit in a different way. This suggests that she only came to her senses after being resurrected.]].
* In an episode of ''NatsumeYuujinchou Shi'', Nyanko-sensei has to masquerade as Natsume after he's trapped in a bottle by a youkai. Nyanko-sensei then proceeds to act exactly as he usually does (read:a {{jerkass}}), much to Natsume's dismay.
* In one ToLoveRu story, Haruna suddenly became an evil sadist, which was hilarious. It became a FunnyAneurysmMoment when the artist's wife, who Haruna's design was based on, turned out to be a crazy psychopath who kidnapped their daughter, among other demented things.
* Played for drama toward the last chapter of the Golden Age Arc in ''{{Berserk}}'', which concludes the prologue that dives into Guts' backstory about [[HowWeGotHere how he came to be]] the dark, brooding and vengeful man [[EstablishingCharacterMoment that he was introduced as]] in the Black Swordsman Arc. Although they were not present at the harrowing Eclipse [[spoiler: a tortuous event triggered by Griffith, his former friend, when he [[FaceHeelTurn turned to the dark side,]] ending with the sacrifice of his comrades and the rape of his lover]] Godo, Rickert, and Erica noticed the dark shift in Guts' character, which not only involved the increased blood-lust in his personality, but also his dark dress and intense scowling. Even the audience can notice a clear change in Guts' appearance just chapters apart.
* A frequent occurrence with Yuu from ''Manga/{{Holyland}}''. More experienced fighters are able to recognise when his heart is not properly in the fight, as his skill takes a corresponding drop.
* Episode 7 of ''Anime/KillLaKill'' has Ryuko noting how [[Cloudcuckoolander Mako]] becomes less and less like her usual chipper self after becoming the Fight Club president. First she becomes increasingly formal and smug, and when she gets a Two-Star uniform, she is noticeably more brutal and doesn't even crack a smile.
** Episode 21 has Ryuko behaving highly sociopathicly following [[spoiler:being both brainwashed by her mother.]]
[[/folder]]

[[folder:Comic Books]]
* Cosmic Boy of DC's post-Zero Hour ''Comicbook/{{Legion Of Super-Heroes}}'' spent a whole {{Arc}} being Not Himself and alienating his teammates at every opportunity [[spoiler: in order to set up an elaborate EngineeredPublicConfession which revealed the President's corruption and caused her removal from office]]. Some Legion fans never forgave him for it. (It didn't help that this particular arc occurred in the first couple years after a total reboot of the Legion, so it wasn't as clear as it might have been that this wasn't just Cos's new personality.)
* Apparently, hacking into {{Superman}}'s mind is like evil hypnotists' [[WikiVandal wikivandalism]]. Some bratty six-year-old in Minneapolis trips over a ouija board, next thing you know our Man of Steel is giving the Batman a wedgie. Supes has probably been brainwashed, hypnotized, or possessed more than any character in fiction. And if you count the times he's [[SuperDickery pretended]] to be NotHimself, more than all fictional characters combined. Read any random five Superman or Superman-associated comics from the 50s through the 70s, and see if you can't find one of these stories.
** Or refer to [[http://www.superdickery.com/ here]] for the condensed version.
*** Since this is practically Superman's only vulnerability besides kryptonite, writers got stuck using it lest he become a BoringInvincibleHero
** There's also the arc in 2000 where Lois and Clark's marriage appeared to start falling apart, with Lois accusing Clark of not caring about her because she was just a human, and of cheating on her with Wonder Woman, effectively turning the poor guy into a miserable wreck and culminating in her walking out on him. Turns out it was actually Parasite masquerading as Lois in order to simultaneously feed off of Superman's powers and destroy his spirit in a plot to kill him. The arc wound up with what looked like [[CrowningMomentOfFunny a superpowered-Lois Lane beating the snot out of Superman right in front of a flabbergasted Perry White and Jimmy Olsen]], and apparently required loads of explanations from the real Lois after everything was cleared up, especially as Parasite had been running around using Lois's know-how to steal millions of dollars.
* In the ''JusticeLeagueInternational'' comic, Guy Gardner spends a rather extended period like this after minor head trauma. The abrasive, chauvinistic, arrogant Guy turns into a sweet, Sensitive Guy -- who really exists deep down, as exhibited when Guy falls in love. This trope is partially subverted, though, because the League ''likes him better as Sensitive Guy'' and therefore makes no effort to return him to his previous state.
* During the "Trinity" arc, it is revealed that Superman, Batman and Wonder Woman are "Keystones" of reality, inherent to the fabric of the universe, and that the endless Expies of them across the multiverse are the "Keystones" of their universe (including [[TheAuthority Apollo, Midnighter]] and [[WildCATs Zealot]] for the {{Wildstorm}} universe, whch makes... [[{{Grimdark}} a lot]] [[NinetiesAntihero of sense]]). When a trio of villains tries to usurp their position for ultimate power, the spell they use has the side effect of of causing them to take on each others personality traits, meaning on a mission to the MirrorUniverse, Superman goes LeeroyJenkins and [[CurbStompBattle Curb Stomps]] the [[EvilTwin CSA]] while Batman tries to negotiate a peaceful solution and Wonder Woman broods on their out of character behaviour. Then when Superman sends Ultraman, Owlman and Superwoman out of their universe, it starts trying to force them to become the new Keystones, and they start acting like their CSA doubles as well as each other. Fortunately ComicBook/BlackCanary tells them to go home and get their act together.
* ''[[Comicbook/ArchieComicsSonicTheHedgehog Sonic The Hedgehog]]'': After the TimeSkip, Antoine inexplicably became an insufferable JerkAss. Writer Karl Bollers wanted to write this off as natural character growth, but fellow writer Ken Penders decided he [[ArmedWithCanon didn't like that idea]], so he changed to Antoine having been switched with his EvilTwin from an AlternateUniverse. And about the time of that reveal, Sonic's own twin from that same AU switched places with him as well, so the other Freedom Fighters had to deal with "Sonic" acting out of character too.
[[/folder]]

[[folder:Fan Fic]]
* ''MyImmortal'': The only reason Dumbledore swore was because he had a ''headache'', okay? (Of course, he then swears regularly for the remainder of the "text".)
* ''Fanfic/ADelicateBalance'': [[WesternAnimation/MyLittlePonyFriendshipIsMagic Pinkie Pie]] immediately realizes that Twilight has something on her mind when the Twilight fails to be an utter perfectionist when decorating cupcakes.
* In the ''StarTrek'' fanfic ''FanFic/{{Retribution}}'', Spock convinces Kirk to forgive himself for [[Recap/StarTrekS2E12TheDeadlyYears his harsh words]] by forcing him to admit that the RapidAging was affecting his mind.
* In ''Fanfic/ThoseWhoStandForNothingFallForAnything'' Takada regularly excuses Light's irritabile moods by telling everyone he has a headache.
* In ''FanFic/MegaManDefenderOfTheHumanRace'', [=ProtoMan=] is this post episode 10; the other characters begin to notice midway through episode 11.
* The ponification potion in FanFic/TheConversionBureauTheOtherSideOfTheSpectrum completely takes away the victim's original personality, effectively killing them. War March and Past Sins delves into exactly why this is - [[spoiler: basically the human convert's soul is shattered to pieces and then "bound in chains" to Queen Celestia and the Bag of Tirek's will. Even worse, according to the Elder Shaman of the Zebras, it is completely impossible to repair those souls with any medicine or magic; only Mercy Killing them will make their souls whole again and set them free.]] To quote one ex-HLF member:
-->'''Andrew Silas''': "We expected them to just be our friends and family with hooves and fur, we expected... I don't know what. The simplest way I can put it is that we expected it to be them. That they'd be something like the people we knew. Henry, though - you've heard this description a million times, but it just wasn't him in there."
[[/folder]]

[[folder:Film - Animated]]
* A [[TearJerker tearjerking]] example occurs in ''WesternAnimation/{{WALL-E}}'': [[spoiler: EVE repairs WALL-E after he's been crushed to death. However, once he's started up again, WALL-E has no memories of EVE, and his personality is gone. He's just another trash-compacting robot. EVE especially notices he's not himself when he regards all the trinkets he's collected over the centuries as just more trash, and is completely indifferent to the cockroach he adopted as a pet.]] Fortunately, [[spoiler: he gets better]].
[[/folder]]

[[folder:Film - Live Action]]
* Inverted in ''Literature/DangerousLiaisons'', when the Vicomte Sébastien de Valmont, known to be a ruthless, womanizing manipulator genuinely falls in love with Madame Marie de Tourvel. He is forced into the situation by Marquise Isabelle de Merteuil, and with tears in his eyes has to push away the woman he loves through cruel manipulation. She is aware that it's [[OutOfCharacterAlert odd for the Vicomte to speak thus to her]], but is heartbroken nonetheless.
* In ''[[Creator/WoodyAllen Everyone Says I Love You]]'', one of the characters becomes a Republican to the great surprise (and dismay) of his family. It turns out it was because of some sort of brain problem. Subtle, we know.
* Quite a lot of this in ''{{Fallen}}'': "I wanna tell you about the time I almost died...."
* Definitely seen a lot in ''Film/FaceOff'', when plastic surgery for an undercover operation results in Sean Archer (JohnTravolta) getting the face of Castor Troy (NicolasCage), and subsequently, Castor retaliates upon finding out his face has been stolen by stealing Archer's face for himself. By having Cage and Travolta swap roles, one sees how the two characters react differently when in the company of the other's inner circle. One contrast is how they take victory celebrations: when Archer gets his for putting Castor into a coma, he looks depressed and turns down a bottle of champagne, then decides to remind them of the agents killed by Castor in the shootout. When Castor is being congratulated for "locating" and disarming his own bomb, he wears his sunglasses and seems unusually chipper. Elsewhere, when Castor gets his first glimpse of Archer's daughter Jamie, he inadvertently calls her "Janie", and also lights up a cigarette, two things the real Archer would never do, but which are glossed over.
[[/folder]]

[[folder:Literature]]
* Ditzy but loyal vampire queen [[Literature/BetsyTheVampireQueen Betsy Taylor]] becomes a real bitch after reading too many pages of the [[GreatBigBookOfEverything Book of the Dead]] in a row in ''Undead and Unappreciated.''
* In the VCAndrews novel ''April Shadows'', April's father starts to act like a {{jerkass}} to his family for seemingly no reason. [[spoiler: The reason turns out to be that he found out he had terminal cancer and he didn't want his family to feel sad for him when he died. So, he decides to make them hate him. Once April, her mother, and sister find out, though, they feel sorry for him anyway.]]
* This is one of the main plot points in the ''Literature/{{Animorphs}}'' series due to the fact that the primary villains are aliens who take over the bodies of humans and mimick them near perfectly. [[spoiler: In one book, Jake is taken over. The only reason the Yeerk fails is because of a momentary slipup in which he shouts Andalite Filth.]]
** [[spoiler: That was only the last straw. It was obvious to everyone that something was wrong when as a precaution to have fallen into the pool they suggested that Jake be tied up for 3 days in order to starve any possible Yeerks that might have infested him. The paranoid and cautious Jake would have either immediately agreed or given into peer pressure and reluctantly agreed but since that would have meant the death of the Yeerk controlling him it had him act OOC and continually and desperately try to talk them out of it, which they quickly caught onto.]]
* In L. Jagi Lamplighter's ''[[ProsperosDaughter Prospero in Hell]]'', Miranda admits that she had not noticed that Mab had been replaced with a shapeshifter, just that he was acting oddly.
* Marianne Dashwood in ''Literature/SenseAndSensibility''. After she is jilted she falls into a despair and eventually after months of barely eating or sleeping she stays out for hours in a rainstorm while insufficiently wrapped up. She nearly dies of fever or has a bad flu (YMMV) but when she recovers she has suddenly gained about five years worth of emotional maturity and not long afterward marries someone much older who she was not in love with. Marianne is passionate and talented and struggles to conform to what her society expects of a young unmarried woman. It feels as though Austen is building up to tragically killing her off but at the last minute she backs off and Marianne is suddenly able to deal with the pressures of hiding her feelings in the public space.
* After Tash is injected [[spoiler: with TheVirus]] in ''Literature/GalaxyOfFear: The Planet Plague'', she develops a rash, then a growing bump, and acts increasingly more irritable and angry than is usual for her. She was showing some signs before the injection, but they got worse after it. Turns out that anger makes the disease progress more quickly [[spoiler: and speeding her transformation into a BlobMonster]] - when she made herself calm down and embrace more positive emotions, it helped.
* In ''Literature/ThoseThatWake'', [[spoiler:Brath]] is this once the heroes are trapped in the forest.
[[/folder]]

[[folder:Live Action TV]]
* In ''{{Chuck}}'', the penultimate episode sees Chuck realizing that something is deeply wrong when a [[spoiler:mind-wiped Sarah gets snappish over a massage and then turns away from him in bed at night]]. He convinces himself that it's just the result of [[spoiler:her traumatic escape from Quinn]], but he can't explain [[spoiler:what she's doing with the Intersect glasses in her bag]].
-->'''Chuck''': [[spoiler:I saw the glasses in her bag before we left. I didn't want to say anything because I didn't want to believe it. But deep down I knew it was true. My wife never came home.]]
* In the second season of ''Series/{{Angel}}'', Angel turns on his friends and finds himself without hope.
** Not to mention his role in the second season of ''Buffy'', after experiencing a moment of pure happiness (ie sex) with Buffy caused him to lose his soul.
** Or you could say that almost the entire runs of ''Buffy'' and ''Angel'' are Angelus [[NotHimself not being himself]].
*** Spike and Dru certainly thought so. Less clear what Darla thought.
** Say it with us now. [[IJustWantToBeNormal B]][[GrandTheftMe u]][[CameBackWrong f]][[TookALevelInJerkass f]][[WhatTheHellHero y]][[GeneralFailure .]] She does this even more than [[SplitPersonality Angel.]] Whether it be a spell going wrong and making her fall in love with Spike or the way she cracks under the pressure makes the fans crack at her attitude whether or not the Slayer is herself is, quite literally, largely dependent on how her [[ImportantHaircut hair]] is that week.
** Season 4 of ''Angel'' has [[spoiler: Cordy]] be this after being possessed by the BiggerBad. The end of season 5 has Angel as a FakeDefector in order to stop the Apocaplyse. [[SuperDickery The audience isn't immediately aware of this, though.]]
** Tara suddenly acting violently homophobic towards her girlfriend, coupled with Buffy and Riley disregarding everyone and everything around them to engage in CoitusUninterruptus, clue the Scoobies in as to the nature of what they are dealing with in one episode.
* ''Series/TeenWolf'': Jackson becoming creepily pleasant, albeit increasingly unstable. Allison takes notice.
** A full moon seems to make Scott meaner.
* ''TwinPeaks'' ended with a NotHimself, presumably to have been the plot of the cancelled third season. "How's Annie? How's Annie? How's Annie?!"
* ''Franchise/StarTrek'' has several, mostly involving Kirk and Picard.
** Data is pretty good at them, too. Usually when he's possessed by one thing or another, or something's going weird in his programming.
** Geordi [=LaForge=] gets one of these in the episode [[ManchurianAgent "The Mind's Eye"]]. The most disturbing thing about it being that asides from his being brainwashed and attempting to murder someone in full view, he behaves entirely and utterly ''like'' himself.
** ''Series/StarTrekTheNextGeneration'' basically has three plots: (1) The Enterprise suffers engine failure leading to an hour of TechnoBabble, (2) The holodeck tries to kill everybody, or (3) one or more of the crew members get their minds taken over by aliens. All of the episodes in the 3rd category are examples of this trope.
** In the ''Series/StarTrekTheOriginalSeries'' episode "[[Recap/StarTrekS2E1AmokTime Amok Time]]", Spock's uncharacteristic emotional outbursts are the first clue that something is [[MateOrDie seriously wrong with him]].
* Used regularly in ''Series/{{Farscape}}''. On one occasion it becomes clear that it's not really Aeryn when she doesn't know [[spoiler: she's pregnant]], once when she suggests abandoning their friends, and once when she's suspiciously happy: 'Aeryn smiling for no reason. That ought to have been our first clue.'
** Perhaps the most famous example of this trope, though, is when Harvey takes over Crichton at the end of Season 2 and [[spoiler: forces John to kill Aeryn]].
* ''{{Smallville}}'' has its fair share of NotHimself episodes, no thanks to Red Kryptonite and the truly absurd number of villains who have possession abilities.
* ''TheOffice'', American version: assistant manager (or assistant to the manager) Dwight Schrute, a pompous, anti-social jerk, inexplicably starts acting nice to everyone. Later it is discovered to be the effect of a concussion Dwight suffered as a result of a car crash earlier in the episode.
* In ''Series/BabylonFive'', Michael Garibaldi spends most of season 4 acting strange and distancing himself from the rest of the crew. It turns out [[spoiler: his mind had been programmed to act this way by telepath Bester, who used him to infiltrate an anti-telepath conspiracy. [[MoreThanMindControl Betraying Babylon 5 and Sheridan was an unexpected bonus]]]]. Interestingly, it's revealed that his personality wasn't actually altered. Instead, [[ShadowArchetype the darker aspects of his personality]] were intensified, making him even more paranoid and suspicious of authority than normal.
* ''NewsRadio'': Matthew punched out Bill after one of Bill's pranks went too far. Somehow, the two [[PersonalitySwap switch personalities]] - Bill becomes submissive, while Matthew starts acting like the office alpha male. An accidental slap from Bill reverses the effect, and the two return to normal.
** In a later episode, Matthew suffers a mid-life crisis after turning 30 and starts acting like a '70s-era British punk.
* On an episode of ''{{Kojak}}'', Lt. Kojak appears to go corrupt, but of course in the end it's all a [[FakeDefector big undercover operation]].
* Used to the point of exhaustion on ''KnightRider'', where at least twice a season someone would screw with KITT or manipulate Michael for various reasons.
* ''Series/GetSmart'': [[GeniusDitz Maxwell Smart]] is attempting to infiltrate a KAOS group, and as such needs to dramatically burn all his bridges with CONTROL. Of course, this being ''Get Smart'', it [[HilarityEnsues doesn't work quite as planned]].
* Happens a lot in ''Series/StargateSG1''. Between the Goa'uld, mind control, robots, clones, and other random behavior changes, the characters get a pretty good taste of this.
* In ''Series/StargateAtlantis'', the audience gradually realizes that Sheppard is NotHimself after being fed on by a Wraith/human hybrid when he A) beats Ronen in a race, B) beats Teyla in a fight, C) asks Teyla to call him "John", and D) kisses Teyla. He realizes something is up himself after the last one.
** They get pretty good at spotting it too; in fact, standard SG training involves numerous scenarios that involve teammates possibly being compromised by a Goa'uld.
* In season 5 of ''{{Lost}}'', after [[spoiler:returning to the Island]] Locke is not himself, acting more determined and secretive than ever and knowing things he can't possibly know. Because [[spoiler:he's still dead and The Man in Black is in his place.]]
** In season 6, [[spoiler:Sayid and Claire have been infected with The Sickness, and are now evil recruits of The Man in Black]].
* This happens to almost every member of Torchwood in the ''Series/{{Torchwood}}'' episode "Adam." Especially Owen.
* This happens twice in ''Series/TheXFiles'', once in "Small Potatos" that involved a shape-shifting man, and another in "Dreamland", where Mulder switches bodies with an Area51 worker. Both Non-Mulders try their hand at seducing Scully.
** In "Ice" parasitic organisms cause hosts to act aggressively. "We are not who we are."
** Some crazy planetary and geological vortex in "Syzygy" makes everyone behave weirdly and out of character. Even Mulder and Scully.
** In "Fight Club", Mulder and Scully beat each other very nastily because they encounter two doppelgängerish half-sisters in whose presence everybody gets extremely violent.
* Anytime an angel or demon appears on ''Series/{{Supernatural}}'' could count as this since both angels and demons must possess a human host in order to operate on Earth. Most of the human hosts never appear onscreen without being possessed by the angel/demon, but they obviously wouldn't have their angelic/demonic powers when they're not being possessed.
* Duncan under the Dark Quickening in HighlanderTheSeries
* Basically any ''Series/{{Charmed}}'' episode that focused on possession or the sisters being [[TheCorruption corrupted]] somehow; it's happened to each sister at least once (and to all three at once in at least one episode), let alone their love interests and the supporting cast.
* BeingHuman: When Mitchell is undergoing some SanitySlippage at the end of season two, he starts hitting on Annie and makes her incredibly uncomfortable. Annie runs into the other room and tells George "There's someone in there." George is initially confused as to why she didn't just say "Mitchell" but it becomes clear quite quickly that Mitchell is, in fact, very much NotHimself.
* On ''Series/HouseOfAnubis'', when Fabian and Alfie become sinners, everyone is scared because they are acting like total jerks, when in reality they are two of the nicest people in the house.
* ThirdRockFromTheSun had an episode where orders come through that Dick is to be replaced as mission commander. After the replacement shows up (looking exactly like Dick, and imprisons Dick instead of sending him home) an emergency message comes through that the previous orders were faked and the 'replacement' is actually a renegade who wants to conquer Earth. Unfortunately, the only people around when the warning comes through are New Dick and Harry (who's brain shuts down when messages come through). Much hilarity ensures until the problem is corrected.
* ''Series/{{House}}'': At the end of one episode that deals with a sociopathic patient, the patient tells her husband (who had only just found out about her sociopathy) that she never loved him and never wants to see him again. This leads House to realize he was correct in believing that the symptoms for which she was hospitalized were caused by an underlying condition that also caused her sociopathy. He calls her out on the fact that she had nothing to gain by leaving her husband, as she had only married him for his money in the first place. She had actually said what she said because her newly discovered conscience wouldn't allow her to keep using him, whereas if the treatment had not cured her of her sociopathy, she would have pretended otherwise in order to keep him around.
[[/folder]]

[[folder:Music]]
* The Brandi Carlile song "That Wasn't Me" is about how addiction can change how a person acts and cause them to hurt the ones they love and do things they're shamed of.[[note]]WordOfGod says the song was inspired by a friend's battle with addiction.[[/note]]
-->"Whatever you've seen/That wasn't me".
[[/folder]]

[[folder:Toys]]
* Hafu starts to notice that [[KidAppealCharacter Lewa]] is behaving strangely at one point in ''{{Bionicle}}''. This was the result of Lewa [[GrandTheftMe having his body stolen]] by Tren Krom via a FreakyFridayFlip. The real Lewa was stranded several hundred miles away in the body of an EldritchAbomination which was [[AndIMustScream fused to an island.]]
[[/folder]]

[[folder:Video Games]]
* [[spoiler: Zidane]] in ''VideoGame/FinalFantasyIX'' displays this for a short while near the end of the game. Given his normally cheerful personality, this moment is notable as it hints at [[spoiler: what he was really capable of if he had been acting towards his intended purpose -- i.e., an angel of death and destroyer of worlds]].
** A good chunk of the game's first half is driven by Princess Garnet wanting to find out why her mother was not acting like herself. Turns out she was being influenced by Kuja.
* In ''VideoGame/FinalFantasyIV'', it is said at the beginning of the game that the King's behavior changed. [[FakeKing Of course...]]
* [[VideoGame/KnightsOfTheOldRepublic HK-47]] can have a Not Himself moment in the second game if the player installs a Pacifist Package into him. Needless to say, [[CrowningMomentOfFunny this genuinely scares the hell out of him]].
* In TheLegendOfZeldaMajorasMask, one of the sidequests you need to complete for OneHundredPercentCompletion is helping a little girl, Romani, with defending the ranch's cows from aliens on the night of the first day. If you fail, the aliens abduct the cows and Romani, since she was also in the barn. They bring her back on the third day--but she can't think straight, doesn't recognize Link, and shakes her head, trying to remember or forget things. It's assumed by the whole fandom that she was lobotomized.
* Video games in general bring a unique version of this where the out of character behavior is caused by the player controlling the action either being a jerk or having a sense of humor toward the choices the game throws at him.
** ''FinalFantasyTactics'', in an early mission, you find a man being menaced by bad guys. You have a choice to either prioritize beating up the bad guys, or saving their victim. Choosing the former prompts the main character's best friend to ask if he's flipped his lid.
*** [[spoiler:Choosing this option actually makes the mission easier though, as your goal becomes merely to defeat the enemies rather than save the NPC. The NPC is sometimes suicidal, and will live even if knocked out, so there's no loss to doing so. Said NPC is also a [[AssholeVictim colossal jackass]], so repeat players can only wish he could die permanently.]]
** SuperPaperMario, there's a funny mini-game where you play as Peach trapped in a dating sim and can choose her reactions to the other character's fumbling attempts to court her. You can of course choose to have her react favorably... An interesting case in that doing this repeatedly will cause the princess to call -herself- on her bizarre behavior.
* In episode 105 of ''VideoGame/SamAndMaxFreelancePolice'', you find yourself in a virtual world where four computers are in charge, each one controlling a certain setting (pop-ups, height control, gravity and three-dimensional view). In order to progress you need to place a bug on each one of them at one point or another, with the side effect of completely reversing their personalities. An old Atari console who communicates through beeping and booping begins playing a melody, a smarmy, conversational telephone's voice gets hacky and hoarse, an aggressive arcade machine starts belting out showtunes and an outdated Osborne 1 starts spouting youth slang.
[[/folder]]

[[folder:Visual Novels]]
* In ''VisualNovel/FateHollowAtaraxia'', the fact that many of the characters are nothing like they were in ''VisualNovel/FateStayNight'' is actually an early clue that something's wrong with this reality besides the whole GroundhogDayLoop thing.
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[[folder:Web Animation]]
* ''WebAnimation/{{RWBY}}'': When Professor Ozpin gives the induction speech to the academy's new students, his speech is oddly clipped, one-note and lifeless. He also cuts the speech very short, leaving Glynda to complete the speech. Ruby and Yang both notice something seems to be wrong and that he doesn't seem to sound or act like himself. Ruby even exclaims "It's like he's not even there!". Played for mystery, as the girls have no answers to why he's behaving so oddly and the next time he appears he's his normal self again.
[[/folder]]

[[folder:Web Comics]]
* [[http://comic.nodwick.com/?p=131 Parodied by]] ''{{Nodwick}}''.
* In ''Webcomic/GunnerkriggCourt'', Chapter 18, Robot S13 -- previously the comically {{Pollyanna}} ChewToy -- abruptly begins acting deadly serious and [[TookALevelInBadass proves to be a dashing swords-bot]] after being transferred to a much older body, much to the surprise of Annie and Kat. He returns to normal after his body is changed once again.
** After a trip in Zimmy's twisted mind, Jack began becoming [[GoMadFromTheRevelation more and more unstable]]. He gets better after Zimmy [[spoiler: kills the spider that had him possessed.]] This is an interesting case, however, since the audience only really has a chance to get to know Jack's real personality after he's cured.
* In ''{{Fans}}'', when Alisin's incurable and deadly blood disease starts to kick in, she sets out to pull an April Shadows in order to leave her loving boyfriend less sad when she goes. [[spoiler:Despite (or perhaps because of) the best efforts of the BigBad and the traitor, he not only figures it out but time travels to a period when the disease has been cured thanks to the FIB's efforts to save ''her'', lets a teammate trick the BigBad into contracting the disease, and thus distracts her long enough to get some of the cure for Alisin. Everything works out- well, sort of.]]
* In WebComic/DeadOfSummer, [[spoiler:[[Music/TheProtomen Panther]]]] falls under this. At first he seems normal (and awesome), though breaking a bad guy's finger may raise some eyebrows. But later on, he [[spoiler: [[EyeScream tears out Dr. Light's eyes]], kills him, and proceeds to reveal that he's [[FaceHeelTurn in league with the Big Bad.]]]]
** Turns out [[spoiler: he's ''literally'' not himself; the Panther we'd been following was the EvilTwin. The real one sets things right.]]
* In ''Webcomic/CaseyAndAndy'', {{Satan}} places [[MadScientist Andy]] by the Soul-Keeper, an ancient and complicated machine. She was able to tell later he'd been replaced by [[TheStarscream Azrael]] because he hadn't started taking it apart to see how it worked.
* It [[ThisIsYourBrainOnEvil appeared this way]] with Varsuuvius in ''Webcomic/TheOrderOfTheStick'', but accepting that it ''was'' consistent with his character was part of his CharacterDevelopment. A straight and more striking example would be Belkar's [[SnapBack very brief]] wisdom boost turning him repentant and pacifistic.
* In ''{{Newshounds}}'', Rochelle being ''afraid'' of danger instead of craving it was the first sign of [[spoiler:her pregnancy]].
* In AlienDice, Lexx has one of these events later in the comic. It's rather obvious and jarring, considering his usual "nice but distant" personality.
* In ''Webcomic/{{UC}}'', when [[http://www.wanderingchaos.com/uc/022012.html Naim]] begins to break his normally stoic attitude to talk to Iku about her love life in great length, it's a sign that the world they have entered is messing with his head, and that she needs to be on guard.
[[/folder]]

[[folder:Western Animation]]
* Briefly in the ''WesternAnimation/DannyPhantom'' episode "The Ultimate Enemy" where Danny's [[FutureMeScaresMe evil self]], Dark Danny, goes to the past (present time) and disguises as his old fourteen-year-old self. His friends wonder what's up with him when he starts speaking rather eloquently, but being a [[ConsummateLiar smart bastard]], Dark Danny quickly fixes this and their suspects drop.
--> "Who's up for... what did we used to do together? 'Playing video games and raging against the machine?'" "We're in!"
** Danny also holds the dubious honor of at one point doing this ''twice'' at the same time. An accident splits his personality into the overly dramatic, comic style Hero Danny, and the laid back, mundane (and I quote) 'Fun Danny'. Confusion (and hilarity) briefly ensues.
* ''WesternAnimation/WinxClub'': Stella was acting very rude and antagonistic to the Winx after her first date. It turned out she was being impersonated by Darcy. [=4Kids=] turned Stella being a bit bitchy with her roomies a few episodes later from being part of her character to being the result of a spell.
* ''WesternAnimation/Ben10UltimateAlien'': When Ben has to be in three different places at once, he becomes Echo Echo and then reverts back into 3 of his human forms (one [[JerkAss arrogant]], one "[[AmbiguouslyGay sensative]]", and one [[ManChild immature]].) At first everyone rolls with it, [[NiceJobBreakingItHero until they find out what's really going on]], [[WhatTheHellHero to which they promptly chew him out for it]], leaving for one of the few {{Downer Ending}}s in the series.
[[/folder]]

[[folder:Real Life]]
* Several illnesses or psychological factors can lead to a person's behavior changing dramatically. One of the most well known examples would be Phineas Gage, a railroad worker who was said to have been honest and reliable prior to an accident which lead to a metal pole being ''blasted through his head, damaging both his frontal lobes''. Afterwards, he became a much nastier person. The accident shed light on the effects that frontal lobe damage can have on personality.
** Maybe it wasn't the accident. In the words of the clearly well-researched website Cracked:
-->[...]scientists have come up with all kinds of theories about the damage affecting his impulse control. But, holy crap, some of us turn into dicks after missing morning coffee. Can we not cut a guy some slack after surviving a fucking spike through the skull?
** He wasn't competent to run his crew anymore. He didn't have ''respect'' anymore. Plenty to get mad about.
** He lost his hypothalamus and frontal lobe connection, meaning less stuff was routed through the frontal lobes, particularly emotion.
** Reportedly, Gage started drinking heavily after the accident -- which could be either a direct result of brain damage, or an attempt to dull chronic pain from the injury. In either case, the new personality might be simply be him being a mean drunk.
** He was also reputed to have taken to gambling as well as drinking after the injury. The part of his brain that was damaged is also involved with planning and predicting future events, so he may have become reckless, in part at least, as a result if impaired ability to appreciate the consequences.
* On the other end of the brain damage spectrum, it's possible to damage the part of your mind responsible for recognizing people. Not that you'd forget who they were, mind you. You'd know who your mother is, exactly what she looks like, but your mind just won't recognize the person who acts like and looks like your mother as actually being your mother. People with this type of brain damage believe that everyone they know [[InvasionOfTheBodySnatchers has been replaced with an imposter]]. All of a sudden, everyone else in the world is Not Him/Herself.
* Lobotomies. Goddamn lobotomies.
** Which ''starts'' by ramming an ice pick ''[[EyeScream into your eye]]''. Hey! Where are you going? I only got started!
** Rosemary Kennedy (sister of former U.S. President John F. Kennedy) was having mood swings and underwent a lobotomy at age 23, which left her permanently incapacitated. After the experience her speech was unintelligible, she would stare blankly at walls for hours, and was left mentally handicapped.
* The US first learned about the Soviet plan to [[CubanMissileCrisis put nuclear missiles in Cuba]] when their spy planes captured a bunch of new soccer fields being built, since the most popular sport in Cuba is baseball, applying this trope to an entire country.
[[/folder]]

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