A Flat character is usually the minor character in a story. These characters are in a work of fiction who do not tend to undergo substantial emotional change or growth. Flat characters are also referred to as “two-dimensional characters,” and are the opposite of round characters.

Round or multi-dimensional characters are very complex for the most part – they are the interesting folks who keep the story moving. Remember the story “Misery”? Annie was written as a compassionate, understanding person, but she was also a killer. You never knew what she was going to do – Flat characters usually don’t change. But round characters are always going through motions and challenges to keep the plot of the story moving.

A “flat” character would only have one main characteristic: a bad guy who’s just always bad, no redeeming qualities at all, etc. Flat characters are very predictable because you always know what they’ll do. The bad guy is always going to do the worst thing possible. A multi-dimensional character isn’t just black or white; there are shades of gray to the character. Maybe a villain who was mistreated as a child, who has a soft spot for certain types of people, or who is capable of loving some one. Still a bad guy, but with a more-rounded character, just like real people. Very rarely is someone all one thing or the other; people are combinations of traits.

You mean in fiction? (Why’s this in Trivia?)

A flat character exists only as far as his function in a story. There’s nothing to suggest that there’s any more to him than his role propelling the story forward. Minor characters are often flat without it being a problem–think cops, waiters, reporters.

A multi-dimensional character has an actual life, opinions, dreams, a past, hopes for the future, things he hates, people he loves, a family, yada-yada-yada, often not spelled out but implied through his actions, dialogue, and interior monologue if his point of view is used. The reader could make a decent guess on what he does weekends, or any time he’s “off camera” as far as the book goes.

Flat chararcters are like the letters you’re reading now, they seem to be lying flat on a surface. Multi-dimensonal characters appear to be raised up, as if embossed, or somtimes even more so… like a movie, more ‘realistic.’

EDIT: If you meant story characters, all I have to say is a somewhat embarrassed “oh.” The others have explained. Wish you had made it clear. No matter, it’s not the first or last time I’ll misunderstand.