To keep the character consistent.
It's hard because events could swallow up a character.
For example (weak one) In the play the little ones are doing... takes place during the roaring twenties. There are bootleggers and crooked cops, etc. Well, one bootlegger has two "hit men" that work for him. Virgil and Lester. The joke is that Virgil and Lester are the two smallest characters on the stage. That joke works and then the joke is over. Except everyone has to keep being afraid of Virgil and Lester. No matter how incompetent and little and cute they are. I find my "actors" forgetting this, but they can't. Until the end, the dread of the stage is that Virgil and Lester will rub you out.