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[[quoteright:189:[[WesternAnimation/ThomasTheTankEngine http://static.tvtropes.org/pmwiki/pub/images/s_flanderization_6275.png]]]]
[[caption-width-right:189:This engine [[VisualPun derailed]] in a whole new way. [[note]]Top: [[WesternAnimation/ThomasAndTheMagicRailroad 2000]], Bottom: [[WesternAnimation/CallingAllEngines 2005]]. No official HeelFaceTurn took place.[[/note]]]]

->''“Pardon me, but do you mind if we assassinate your character just a little bit?”''
-->-- '''''Website/TheAgonyBooth''''', ''Film/StarTrekVITheUndiscoveredCountry''

When an established character becomes largely different, exhibiting behavior contrary to what has been previously shown. This is not a matter of [[CharacterDevelopment organic growth]]. Rather than gradually changing in response to events and experiences, a derailed character will exhibit shockingly unusual behavior that implies [[StealthParody malfeasance]] or incompetence on the part of the writers.

Note that organic growth does not necessarily mean 'benign growth', and it is perfectly possible for a previously good-natured character to end up embittered or depressed without falling victim to this trope. This is [[DownerEnding rare]] however, and often unpopular.

In published fiction, derailment is usually caused by an author having a certain story idea and needing to stretch the characters' personality to fit. In the worst cases, the author [[AuthorFilibuster may end up speaking]] ''for'' the character. In works with multiple authors, a new author may have a [[AlternateCharacterInterpretation wildly different interpretation]] of a character, causing him to write his version of the character instead of the one established by canon.

Fans motivated by DieForOurShip will use Character Derailment in their {{fanfic}}s to remove the character from the OfficialCouple which is in the way of their {{OTP}} (or just because they dislike them).

In the rarest of cases, this can be an example of how TropesAreNotBad. Previously boring or irritating characters may actually be ''improved'' if they're drastically changed. A classic example is Kelly Bundy from ''Series/MarriedWithChildren'', whose transformation into TheBrainlessBeauty also coincided with the show's GrowingTheBeard.

Please check if examples fit better into subtropes before adding them here.

'''Specific types:'''
* AesopAmnesia
* BadassDecay
* {{Chickification}}
* ComedicSociopathy (for characters who [[SadistShow aren't supposed]] to [[KafkaKomedy act like this]])
* ConflictBall (when characters are derailed just for the sake of drama)
* DerailingLoveInterests
* DorkAge (often involves this trope as well as other [[BadWritingIndex bad writing tropes]])
* FauxAffablyEvil (When the character [[TookALevelInJerkass wasn't that much of a]] [[{{Jerkass}} jerkass to begin with]])
* FaceHeelTurn (When poorly executed)
* {{Flanderization}}
* HeelFaceTurn (When poorly executed)
* HeelFaceReturn
* JumpingOffTheSlipperySlope (if not conveyed properly)
* LoveMakesYouDumb
* LoweredMonsterDifficulty
* MessiahCreep (when unintentional or overdone)
* MoralEventHorizon (when unintentional)
* MotiveDecay
* NotAsYouKnowThem
* OutOfCharacterMoment
* PossessionSue
* SameCharacterButDifferent
* StrangledByTheRedString
* SympatheticMurderBackstory (when handled badly)
* TookALevelInDumbass
* TookALevelInJerkass
* VillainDecay
* {{Wimpification}}
* {{Xenafication}}
* YouDontLookLikeYou

If in {{Fan Fic}}s, it can involve CanonDefilement, DracoInLeatherPants and/or RonTheDeathEater. When the derailed character's characterization is fixed, it is CharacterRerailment. Contrast CharacterizationMarchesOn, in which a character changes because their previous characterization was only a "work in progress". See also CharacterCheck, where a derailed character might briefly act like they used to.

[[noreallife]]'''Administrivia/NoRealLifeExamplesPlease''' Though how it can be applied to {{real life}} is anyone's guess...

* CharacterDerailment/AnimeAndManga
* CharacterDerailment/ComicBooks
** CharacterDerailment/XMen
* CharacterDerailment/{{Fanfic}}
* CharacterDerailment/{{Film}}
* CharacterDerailment/{{Literature}}
* CharacterDerailment/LiveActionTV
* CharacterDerailment/NewspaperComics
* CharacterDerailment/ProfessionalWrestling
* CharacterDerailment/TabletopGames
* CharacterDerailment/VideoGames
* CharacterDerailment/WebAnimation
* CharacterDerailment/{{Webcomics}}
* CharacterDerailment/WebOriginal
* CharacterDerailment/WesternAnimation