What makes a character real?

I share a love for Jan Karon's novels with my mother. We can talk about the characters as if they were real, asking each other why this and how that, and generally driving the male members of our family crazy. But what makes these characters seem so real to us? Perhaps it's the fact that we've read about them so often, seen them grow up or grow old through so many books, enjoyed their conversations with each other and remembered pithy quotes... But how does that apply to other books where the...