Characters. The people who drive our books and stories. The people in our heads who demand to have their stories told.
I think it's important to make sure your characters, while having a life of their own, have very realistic tendencies so they'll read believably. You have to give them a bit of humanity, especially if you're writing in the paranormal genre. Case in point, one of my heroes, while a werewolf, is deathly allergic to broccoli. Why? It makes him vulnerable and makes him identifiable.
On the other hand, characters need to be bigger than life. Someone a reader would root for and want to see succeed no matter what.
Bottom line? Don't write your characters so they come off flat. Give them a life of their own and they'll take care of the rest.