When I was a kid, I thought movies just came from air. I thought they just appeared.
If you ever go to a music session, you'll notice that the musicians can sit down and start playing right away, and everyone knows what to do. Of course they're reading it, but the conductor can tweak little things, and you can take that back to directing motion pictures.
If you're doing a biography, you try to stay as accurate as possible to reality. But you really don't know what was going on in the person's mind. You just know what was going on in the minds of people around him.
My whole life has been one big improvisation.