They're the foundation on which your story will rest. Whether they're witty or charming, evil or innocent, you have to find a way to make your characters come alive. The best way to do this is to, simply put, give them a life. Here's a list of basic things you should know about each of the characters in your book so that you'll know what motivates them and how they'll react under any given circumstance.
Distinctive features (moles, freckles, scars, lisp, stutter, etc.):
Short and long-term goals (this will affect motivation):
So fill out this list for every character before you begin your book. Things can always be added or changed but this will keep you consistent throughout.