Interview the character as if you were a journalist. As the character, answer the questions as to why you did a specific thing or made a certain decision. For instance, if you were to use the character Edward Rochester from Jane Eyre, you might ask him why he never considered telling Jane about his wife. As Mr. Rochester, you may answer that you simply wanted to keep the purity of Jane separate from the rest of your life, which was undesirable to you or perhaps you felt guilty about leaving your wife where she was and didn't have the heart to tell Jane the truth. Use the information you have from the book to determine the character's answers.
List the character's traits -- positive and negative -- and determine how she got each one. Did a specific event or series of events cause the character to adopt those traits? Did the character inherit certain traits from relatives or family? Listing character traits can help you determine the character's motivation for specific actions.
Create a series of random actions and write them on a sheet of paper; these can be anything from punching the character to giving him a cake. Next, write how specifically the character would respond to each action. Does the action provoke a response in the character that is different from what would be considered normal? What in the story makes you think the character would respond it this particular way? Consider each action, the character's reaction and the reasoning behind to further understand the character.
Write down of several character traits that your character possesses. For each of those traits, write three direct actions and three indirect actions from the story illustrating that specific character trait. Write quotes from dialogue the character says in the story and quotes from other characters about him that illustrate the traits.