Some words of wisdom from John Gardner, whose The Art of Fiction: Notes on Craft for Young Writers is the writing book I most often turn to. This one, though, is from On Becoming a Novelist (also good!):
Character is the very life of fiction. Setting exists so that the character has someplace to stand, something that can help define him, something he can pick up and throw, if necessary, or eat, or give to his girlfriend. Plot exists so the character can discover for himself (and in the process, reveal to the reader) what he, the character, is really like: plot forces the character to choice and action, transforms him from a static construct to a lifelike human being making choices and paying for them or reaping the rewards. And theme exists only to make the character stand up and be somebody: theme is elevated critical language for what the character’s main problem is.