[Oliver_W]

Why does he need to leave? Billy Flynn's dance routines aren't overly complex, couldn't the directors/coaches do it with him in a Task Room that's not in use?

I've worked on a couple of shows, and a Big Name was in one. His understudy was in the rehearsals all the way up to the night before, while he was being coached through it separately. Though his role didn't dance at all, so he wouldn't necessarily need to know the choreography.