When an established character becomes largely different, exhibiting behavior contrary to what has been previously shown. This is not a matter of organic growth. Rather than gradually changing in response to events and experiences, a derailed character will exhibit shockingly unusual behavior that implies 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 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 may end up speaking for the character. In works with multiple authors, a new author may have a 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 Die for Our Ship will use Character Derailment in their fanfics to remove the character from the Official Couple 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 Tropes Are Not Bad. Previously boring or irritating characters may actually be improved if they're drastically changed. A classic example is Kelly Bundy from Married... with Children, whose transformation into The Brainless Beauty also coincided with the show's Growing the Beard. Another case of this being a good use of the trope is when a character is clearly not acting like they normally are due to trauma or insanity.
Please check if examples fit better into subtropes before adding them here.
- Aesop Amnesia
- Badass Decay
- Comedic Sociopathy (for characters who aren't supposed to act like this)
- Conflict Ball (when characters are derailed just for the sake of drama)
- Derailing Love Interests
- Dork Age (often involves this trope as well as other bad writing tropes)
- Faux Affably Evil (When the character wasn't that much of a jerkass to begin with)
- Face–Heel Turn (When poorly executed)
- Heel–Face Turn (When poorly executed)
- Heel–Face Return
- Jumping Off the Slippery Slope (if not conveyed properly)
- Love Makes You Dumb
- Lowered Monster Difficulty
- Messiah Creep (when unintentional or overdone)
- Moral Event Horizon (when unintentional)
- Motive Decay
- Not as You Know Them
- Out-of-Character Moment
- Possession Sue
- Same Character, but Different
- Strangled by the Red String
- Sympathetic Murder Backstory (when handled badly)
- Took a Level in Dumbass
- Took a Level in Jerkass
- Villain Decay
- You Don't Look Like You
If in Fan Fics, it can involve Canon Defilement, Draco in Leather Pants and/or Ron the Death Eater. When the derailed character's characterization is fixed, it is Character Rerailment. Contrast Characterization Marches On, in which a character changes because their previous characterization was only a "work in progress". Contrast with an Ultimate Universe, where changing the characters is the very premise of the work to begin with, and do not count as examples (unless, of course, the example is about changes within the ultimate universe itself). See also Character Check, where a derailed character might briefly act like they used to.
- Anime and Manga
- Comic Books
- Live-Action TV
- Newspaper Comics
- Professional Wrestling
- Tabletop Games
- Video Games
- Web Animation
- Web Comics
- Web Original
- Western Animation