I'm going to tell you the story about the geese which fly 5,000 miles from Canada to France. They fly in V-formation but the second ones don't fly. They're the subs for the first ones. And then the second ones take over - so it's teamwork.
I don't know how it is now but the assistant stage manager had to understudy several parts. You had to be ready to go on at any time if the actor couldn't make it to the play. I didn't think anything of it.