You start with a shadow – your character’s appearance and a few personality traits. You give them a goal and a story problem. But they lack dimension, flesh and blood . It's only as you write that you begin to layer motives, back story, complexities and so on until they’re as real to you (and your reader) as family. How do you find those layers?
First, take your time.
As you live with your characters, write them, daydream about them they become more real to you, and thus more real to your readers. This is why, in my not so humble opinion, the most memorable characters belong to books that took a few years to write. Six months isn’t enough time to become true friends. Over two or three years though you can really get to know someone.
Put yourself into their skin.
Imagine yourself in your character’s place. How would you feel going into battle or giving birth? What would you do? Of course, you may not be the shy son of a Revolutionary era Boston merchant or a take-charge executive trapped in a snowbound cabin. If you were though, how would you feel about going into battle or giving birth? Raise the stakes, add in complications and then how would you feel?
What is your character’s past, even if you won’t be writing about it? What are their hobbies, favorite books and songs? What angers them or brings them joy? The more questions you ask, the more the character will take shape.
Find a role model.
Donald Maass advises finding a famous person who exemplifies a central trait of your character. I’ve found though that it’s only after I finish a story that I can see I’ve unconsciously given my main characters elements of people I’ve known – childhood buddies, grandparents or ex-boyfriends. Once I see that connection, it’s helpful to think about the real person. What motivated them? How did they make me feel? What were their mannerisms? What was left unsaid between us? Even if you’ve unconsciously channeled your college roommate, Susan, your character isn’t going to be Susan. Your character will have a number of other traits and a different story. But thinking about Susan will give your character depth.
Find an opposite trait.
Is your character generous? Find a selfish action or motivation for them. Are they brilliant? Find gaps in their knowledge or put them in a situation where they feel stupid. No real person is always true to one trait. So developing a heroic character’s cowardice will make him multi-dimensional
Look for surprises.
Real people do unexpected things, and certainly people we want to read about do. Brainstorm your character’s next action, motivation or spoken line. Keep brainstorming until you’re out of ideas and then come back later when you’re ready to look for more ideas. Chances are the more interesting ideas will be at the bottom of your list.
Intensify your character.
Interesting people are also extraordinary in some way. Even a mousy accountant can become interesting if you can find one quirk and magnify it. When I wrote my character, Sierra, she was a brilliant polyglot. But to make her more interesting, I intensified that characteristic to the point she struggled to function in ordinary life. I inserted all-nighters as she drove herself to learning new languages. I made the sensations she felt as she tried new words on her tongue tangible for the reader, and had her skipping and failing at school in her single-minded pursuit of fluency.
Find a critique group.
A good critique or two can really help, as you write and I suggest giving it another go-round before you submit to your agent/editor. My critique partner points out where the emotion or motivation is muddy and she frequently suggests tweaks to make those places a little clearer. This not only helps me strengthen the character in my own mind, it also helps me get the character in my mind to take shape for the reader.