I can't tell you how many countless times a story with good potential has fallen flat because of bad characters.
Flat characters aren't always a bad thing, don't get me wrong. But when all of your characters, especially your major characters, are flat then something is wrong. Characters should develope with the story naturally. Major characters are often faced with conflicts or decisions and have to make choices reflecting their personality - over time, the story also changes or reveals bits and pieces of their personality. This is referred to as character development.
Flat Characters V.S. Round Characters
Seventh grade English class tells us that a Flat Character is a character that is basically unknown - they don't get character development. We don't know much about their personality. Their motives and methods are left to the imagination. Usually, this is for minor characters whose personalities aren't important to progress the story.
Then there are the opposites: Round Characters. We know these characters. We know their motivations and why they do things - we know what they would think to do in certain situations. Again, this is for major characters.
Too many of either type can jeopardize your story. Too many flat characters make the story bland and tasteless, especially when major characters end up flat. Too many round characters can make the story too long and drawn out, and cause unnecessary details.
Why Major Characters should mostly be Round Characters.
Let's take a look at two different characters: Jeff the Killer, and the Joker. If you read Jeff the Killer, Jeff is flat. His motives are solely being sadistic for the sake of it. He's insane for almost no reason, and what reasons there are he brought on himself. He is a weak character who gets knocked in criticism constantly. Now let's look at the Joker. We know why he's so constantly obsessed with Batman - because Batman thwarts his plans so constantly that he has to get rid of him. He's given a backstory from which we can work. The reason he's such a brilliant villain is because he's well-thought out. His insanity, his actions, his reasoning - they're all justified to what his character is supposed to be.
Major characters need their justification. They need character development to be likable. If it's a protagonist, you want the character to be someone that you'd feel sorry for. We need to get to know who the character is. When it comes to an antagonist, you want to put in reasoning behind why he's against the heroes. Mystery only stretches so far.
Showing and Telling: Personality
There's been a few stories I've seen where we get informed that a character is kind, sweet, and good-hearted - then he turns around and rudely tells a poor man how he should get a job instead of begging for his quarter. There are about a million reasons I can list as to why this is a horrible decision. If you're going to tell us their personality, then make them act like you've told us they would act. Better yet, don't tell us their personality. Let their personality flow through their actions.
Does it sound like a good character to you? Don't tell us they're good. Show. Don't tell.
Recap and Summary
- Don't have too many flat or round characters
- Major characters should mostly be round
- Personalities of characters should be shown, not told.