A topic that has come up in my classroom as well as my writing group lately is how one creates a believable, well-rounded character. Now, granted, I'm not the "end all and be all" in writing, so take my suggestions with a grain of salt, but I'd like to think my characters have depth and dimension, and here's how I go about creating them.
First off, in my opinion, a writer should know far, far more about his or her characters than the mere information that makes it into the finished manuscript. This knowledge adds layers to a character's personality and actions. It gives the character reasons and motivations for what she does.
Far too often, I read a character in a work in progress that does things seemingly randomly. Usually this happens because the AUTHOR needs for it to happen, and not because it is something that character would normally do based on what we know of the character so far. It doesn't work. If I can't accept a character's action, I can't accept that character. Either the action needs to change, or the author needs to include the character's motivations for doing the action.
Think of it like being a method actor. The actor is performing a part in a play that might show only a small slice of a character's life, but the actor might create background for that character in his head so that his emotions ring true when he portrays the character on stage. He tries to expeience what that character has experienced, tries to live the way that character might have lived. It needs to be THAT REAL to the audience.
It needs to be THAT REAL to the reader.
In my manuscript, we might not meet my protagonist until she is twenty-four years old. But I know that character's childhood inside and out. I know her childhood fears and joys, her desires, even details like her favorite colors and foods. Again, these facts might never make the final draft, but hints of them will. Subconsciously, if you know these details, they will find their way into the writing. The character might think back on these fears and desires when faced with something similar as an adult. It gives the character depth.
Now, granted, sometimes I take this character development to extremes. When I'm really into the writing, I will try to think like my protagonist, respond as she does, view the world as she might view it. Writing assassin characters, I can't sit in a restaurant with my back to the door without feeling like I have a target painted between my shoulderblades. I always try to go for the corner seat, much to the amusement of my husband and anyone else who happens to be dining with us.
Another component to good characters is what they say. Speak your character's dialogue out loud. Does it ring true? Does it sound like something a real person might say? Or is it stilted or out of voice for that character? Is it too intellectual in word choice? Too limited?
I recite most of my dialogue out loud to myself before it ever makes it onto the page. Often I'll repeat the same line three or four different ways before I get it the way I want it, the way that character should really sound.
Some writers keep index cards or files on each character. Others do what I do. Whatever method you choose, be consistent and true to your characters' personalities. Characters should be as multifaceted as any person you meet on the street.