There was a play I saw at a Festival in Ireland that I think was by Tom Stoppard about a man who auditioned for a community theatre production of The Three Penny Opera. He was cast in the chorus and eventually ended up playing the lead.
But I can't find a play with that description in the list of his plays. It must be someone else. But it was hilarious.
My favorite part was during a rehearsal the director would get to a certain part in the blocking and then the piano player would start to play. Each time the director would say: "Not yet we're moving on."
He did this about six times. Then the seventh time he got to a song cue and he said "Music, please....." the piano player had gone to the bathroom.....LOL