Sometimes your mind is most free when you're half asleep. To that end, keep a notebook and pen or pencil by your bed. If you come upon an idea rare and useful when you're floating off into the deep waters of rest, force yourself to recover your faculties at once and jot it down. It may be zany, off-the-wall, disconnected, but jot it down. You may discover the best and most unique characters, places, and situations when your mind is unfettered and your subconscious has just begun to revel in its nightly freedom.
Give it a try. Write on!