I usually start with something that has some energy, like a compressed character or a situation that's wound up like a spring. Then all I have to do is let it go, let its energy carry the story. And that may not turn out to be the beginning of the book.
Once you get below the floor of our personal identities, we're all connected. Perhaps that's why we can move into others' lives.
I'm always gratified when I check something I've made up and discover that I've gotten it right. How can we imagine something that turns out to be true? How can we know things we couldn't possibly know? It makes me wonder about the existence of a collective unconscious.