Even the most surreal character needs a grounding in the real world. I base all of my characters on people I've met or know. I can see them very clearly in my mind's eye; the way they speak, move, dress, react to others. Some are extreme versions of people I've met but they still conform to the basic truith of the character. I'm one of those people who likes to create a 'Bible' for each book; a set of rules that govrn the limits of the plot. I create towns and family trees, personal histories and even decide teh decor of houses, the choice of car etc. Even though most if never appears in the book, these kinds of details keep the character real in my head.