I recently read that the main character in "They Live" had an entire backstory that was never told in the movie. Someone (producer, director, ???) Told Roddy Piper to create a backstory for his character and he did, and he played that part even though it was never shared with anyone. I'd thought about that myself for writing - if you define each character ahead of time and keep their character in mind it will aid writing their parts so they are seamless and self-consistent.