Write several questions that relate to the history of the character. These can be as simple as "Where were you born?" or as complex as "Why do you believe your father left when you were 8?" After you write each question, write the character's answer in first person ("I"). Allow yourself to write freely in his voice; you can edit for mistakes later. As the answers come out, you may think of more questions to ask. Continue until you have a good feel for the character's life before the book began.
Write several questions about what happens during the course of the book. These will typically focus on the "hows" and "whys." For example, ask, "How did you feel when you discovered the letter from your ex-wife?" or "Why did you leave the restaurant without paying your tab?" Look through the book for scenes with the character to inspire more questions, and use his answers to come up with more as well.
Write several questions about what the character will do after the book ends. His answers about the future will help you better understand the motivations for his actions throughout the book.
Ask the character to share a secret with you that is not revealed in the book. This can often lead to surprising revelations about the story, even if the secret is not one the author wrote into the book.