For characters to come alive, they must be revealed through the story—through their words, their thoughts, their actions. Through their interactions with other characters.

It's no good telling your reader, "Tom was an angry man," and leaving it at that and thinking that Tom will come across as an angry man. You must portray Tom as an angry man, through the way he talks down to his girlfriend or insults his friend or yells at the policeman who's chasing him down the alley. Or you might do it through his actions: raising a fist, making a rude gesture, slamming a door, or threatening with a weapon.

Don't tell us. Show us.

Make this your mantra; chant it to yourself as you write.

Your characters should evolve through the course of the story; they should change and learn and grow through their life experiences, just as you and I do through ours. If your character changes believably (that is, the way a real person might, and not because you needed him to change for your own convenience), it will deepen the reader's sense of knowing and caring for the character. It's another way of showing, of dramatizing, like an actor on stage.