Remember, though, that power corrupts.
If you don’t know who your boss is on set, then your boss is the Director. This doesn’t mean who gets to boss you around, it means who helps keep you steady-on with your piece of the big puzzle.
If you’re the Director, I’m afraid you’re boned — your boss is the entire movie.
Upon reflection: if you don’t know who your boss is on a low-budget set, then maybe ask the First AD instead of the Director.