Make a list of physical attributes: age, build, coloring, tatoos, scars. You won't have to list off your character's features just because you wrote them down, but it is important for you to have a clear picture of what he or she looks like.
Write down your character's closest relationships: family, friends, co-workers. Even if these character's aren't part of your story -- or make only a small appearance -- your character is shaped by the relationships he or she has.
Where is your character's hometown? Where does he or she live now? Where has he or she lived in-between? Knowing where your character comes from will help you determine his or her language, upbringing, education, and world views.
What is your character's job, role in the family or position in the community? List your character's responsibilities and his or her attitude toward them. This will help you decide how the character will handle situations in the story to come.
What are your character's beliefs? Does he or she have a personal mantra? Is the character driven by a need a change of surroundings because of his or her world views? Write a paragraph on how your character views the world and the people in it. This will help you determine whether or not this is something that should change over the course of the book, or remain true.
What makes your character happy? What makes hi or her angry? What are the character's strengths and flaws? These emotional attributes will help you know how to write your character in his or her best and worst moments. You should have a good idea of how the character will react to changing events. Write scenarios with your character in different awkward or pleasurable situations and see how he or she handles it. These may even be scenes that make it into your story.
What does your character want? What does he or she need? The answers to these questions may not be the same. Your character will be motivated by desires even if he or she doesn't always act on them. Writing a journal entry in the first person on behalf of your character is a good way to know what he or she wants.
Who can help your character find his or her way or help solve a problem? This can be a main character or a passing one, but it is important for you to know who can help your character transform as the story unfolds. This person may be a catalyst or an enemy. Describe the relationship between the character and this other person.
Who does your character trust? This may be a different character than the person who can help. Who your character trusts describes his or her values without having to spell them out.
Is your character telling the story in first person, or are you telling the story in the third person with access to his her thoughts? Either way, you will have to develop her voice. Write a few scenes from your book in both ways, and decide which way is better for the story you want to tell. Does your character have a good outlook on a hard life? Is he or she humorous, jaded or unsuspecting? How do others view him or her? It is important to establish your main character's voice from the first page, to win your audience for the second.
Your character's language should reflect his or her upbringing, education and familiarity with other characters. If you are writing about a certain time period or place, do research on the language and customs of the region and the setting. Knowing the details of your character's speech will make him or her more realistic.
Interview your character when you are stuck for answers. Write a list of questions for him or her and see how they are answered. What does he or she say and what does he or she keep withhold?
Write dialogue between characters -- scenes that reveal how they relate and respond to one another. These are scenes that don't have to make it into your book, but clarifying character relationships beforehand will help you write dialogue with more confidence later.
Make a list of words, interesting vocabulary or phrases that your character uses, especially if your book deals heavily with a cultural setting or fictional place. Keep your list of character phrases handy as you write dialogue.