Whether your characters are drawn from life or are complete figments of your imagination, you should know them as well as you know your best friend, your spouse or yourself. This means that, in addition to being able to describe their external traits-what they look like, how they dress and how they talk-you need to understand what makes them tick, and this means knowing their internal traits-their likes and dislikes, their hobbies, their passions and their personal histories.
An important element of characterization is motivation, that is, what makes your characters do what they do. If you're writing nonfiction, what your characters do (or have done) is a matter of fact, perhaps even of public record. What may not be as evident is why they did it. Introducing your readers to the motivation behind a character's actions will give a nonfiction piece more depth and, ultimately for the reader, more satisfaction.