They usually get a set amount payable on submitting a final draft, or a salary. Which, if it’s an ego project, might be more lucrative than a percentage of profits.

Ghost Writer?

Step 1: become or enthrall a ghost

Step 2: write

I think they’re usually paid by the word. They don’t get residuals.

Or they get to the bottom of things and get run over.

It’s situational. There’s no fixed rule.