I know that I hate nothing more than a character with no past, present or future. Even when I invent my own characters, I can't stand it if they're dead and boring. I give them an interview, and normally end up happy. Ask yourself things like: what is their fear? What is their family like? Are they pressured into doing things they don't want to do? How easily do they laugh? I find when I interview my characters like this, I understand them better. Here's an example with one of my characters.
Name: Rivendale Cloude
Appearance: long, dark black braid that hangs loosely down her back. She has seaweed green eyes and pale skin. She had high cheekbones and naturally thin, pink lips. She has a willowy form.
Personality: she is rather kind. She isn't afraid to voice her opinion or get into trouble. She is willing to stand up to anyone if they threaten her, her family or what she believes in. She give in easily an is very hard to touch. she isn't phased by even a broken bone.
Background: she was a princess, stolen by the Treefolk when she was 5. She had the royalty thrust out of her, and she doesn't have an inch of royalty left in her. She cares more about others than herself.
Ask various other questions, and you have a character that is well plotted out and actually has a life.