What Makes a Character?

I've been wondering lately about what it is that makes a character "real". You know, how some books you read you get so caught up in the characters and actually start thinking of them as a good friend whom you know well. Where as some other books you just cannot get into, you just cannot feel that connection between reader and character. What is it that makes that difference? What do you think? What is it that makes a character "real" to you?