If you are writing a book that has fictional characters, the reader should be able to connect to the characters on a personal level. Make the characters real and interesting, made good descriptions of them, bring them to life. You want a reader to feel like he/she is right there in the story interacting with the characters.
To help make a character, you could model them after someone you know. That makes the characters seem more real because you know what that person would do in a certain situation.