- Give your characters birthdays.
- Write notes or passages about their childhoods, even if they won't appear in the story or novel.
- Think about their ideas on faith, the existence of God, and the afterlife, even if these aspects of the characters won't appear in the story or novel.
- Think of what book(s) they're reading at the time of the story or novel--again, this need not appear in the story or novel, but it can give you an idea about the personalities of your characters.
- Think about their political views.
- Ask yourself, what are my characters' favorite foods?
Again, this is not an exhaustive list--these are just a few of the things I think about when I'm creating characters. I want to know my characters very well, including things about them that may never appear in the work itself. I think that knowing things such as these helps to make them multidimensional on the page. For example, if I know that a given character is a liberal Democrat who's agnostic, these aspects of him may not appear in the work, but they may affect how he reacts in a given situation. If my character loves seafood and is reading The Grapes of Wrath at the time that I'm telling her story, this will help me know her better. These aspects of her may be irrelevant to her story, but I believe that my knowledge of them will show in the work.