Flanderization

The act of taking a single (often minor) action or trait of a character within a work and exaggerating it more and more over time until it completely consumes the character. Most always, the trait/action becomes completely outlandish and it becomes their defining characteristic. Sitcoms and Sitcom characters are particularly susceptible to this, as are peripheral characters in shows with long runs.

The trope is named for one of the examples in The Simpsons, Ned Flanders, who was originally just a considerate neighbor and attentive father, with his devout nature simply being that he willingly attended and paid attention in church, all to make him a contrast to Homer, before becoming obsessively religious to the point of stupidity.

Note that the key to this trope is in how the process is a gradual thing, the character starts relatively normal then gains a few quirks, the quirks become more prominent and then gradually become the character. If it is simply about how the character is different early on before the writers know what to do with them, that is Characterization Marches On. Flanderization doesn't have to be a bad thing - sometimes it can be used to expand on a background character's personality when they are brought to the foreground, or make an otherwise bland character stand out more.

When Flanderization occurs as the result of adaptation from one medium to another (manga to anime, for example), it's Character Exaggeration and frequently a sign of Adaptation Decay. May sometimes be related to Lost in Imitation.

See also Never Live It Down for when the character is more associated with some action or event than the character actually changing, and Unintentionally Sympathetic, when realistic quirks are mishandled by the writers. Compare/contrast Early Installment Weirdness (as it applies to characters), with early depictions of a character being different from later ones simply because the producers hadn't figured out what role they should play in the story.