Choose (or invent) a single incident that is particularly revelatory, a Specimen. It should dramatize not just what the character does, but who he is—what could be going on inside him. You might tell a particularly hair-raising anecdote, like the time Hubert tried to get into the bank through the sewer system, but if the story stays on the surface of the action, what will readers come away with except the sense that this was a very wild guy? It might be better to tell about the time Hubert stole a major chemistry exam for a friend, but wouldn’t look at the exam himself, even though he was weak in chemistry too. That incident seems more evocative, and indicates a character of some complexity. The bank incident seems more exciting, of course, but it has to be told in a way that is similarly revealing.
You have to ask: What kind of understanding do I have of this character? Do I know enough about Hubert’s family and background to say more than he did this and he did that? Do I have the empathy to guess what went on in his head, how he thought and felt about what he did, and what he believed he was doing? And, how do I get that into the story?
A character comes out of a dense cultural, social, and psychological matrix. The more richly this is suggested, the more resonant the portrait. Evocative details about the person’s family, childhood incidents, intimate moments—all are clues that help us understand the character. And remember, too, that you’re writing fiction; you’re creating art. Actual facts are your raw material, not your boundaries.