Whenever a live broadcast of a staged play is planned the final run through rehearsal is recorded just in case there is a technical problem or if a lead actor is unable to perform for the live performance. This common practice is conveniently forgotten in order to create the dilemma faced in this story.
When they are first rehearsing the play in the theater, the boom mic is visible when Perlmutter is asking about his lines.
At about the twenty minute mark, during the argument about replacing the lead actor, the Producer muffs his line by saying, "Don't be serious," followed by the Director saying, "I'm perfectly serious" in his following line. The producer was supposed to have said "Now be serious," without the "don't".