I thought you meant "flushing out," as in getting them to fly up out of the brush so you can shoot them. So I was going to suggest a spaniel.
But seriously, my characters often come to me before the other details of the story. The novel I've been working on started when I was walking my dog along a creek bank, and I had this image in my mind of a young woman walking her dog along the banks of a creek in another time and place (which became my fantasy world) and finding an almost dead guy. So then I got to thinking about what she would do (decided she was a healer who was struggling with her own feelings of inadequacy, because her aunt was the master healer at the university, so everyone assumed the character had gotten into the healing school because of her aunt's influence). But she proves her skill by saving this guy's life. And they become friends, and it turns out he's got dark secrets that he's buried, and they're the reason he was beaten up on the river bank, so then I get to imagine all the possible horrible things he's possibly done.
So some aspects of the history of my world and my magic system slowly emerged as a consequence of needing something dark about someone who was basically a decent person. Something that would make him want to be someone other than the person he was.
At first, I just daydreamed about these people, but finally, I had to start writing. And a lot of things changed once I started hashing the details out. Other characters emerged too. For me, the process of writing (not outlining or making character sheets) allows a lot of those details to come out.
And I've got other characters with problems lurking at the back of my mind besides the ones in this story. And I hope to discover more things about them once I ship this novel.