What do they want. What do they really want. . . and what sort of backstory they have
And how they can come to life, and run off with the story.
One audience member had a problem that he kept inventing backstories to enliven his bit characters, and they kept growing until it seems that he needed a novel for all of them -- except that a new novel would have new bit characters. One-dimensionality is not always your enemy. I brought up my Greek pantheon technique: pick a major Greek god, and assign a trait, such as he's like Ares and so bad-tempered, which does have the danger of bringing them to life. Panelists ran with the notion of using real-life people like that. (The secret to the Greek one is that you can assign the traits from different gods and so orchestrate them.)