The Boy Friend (1971) on Netflix

It's the beginning of the talking movie era. At the Theatre Royal in Portsmouth, England, a second rate theater troupe run by "Mr. Max" Mandeville is in its run of a musical called "The Boy Friend". For their shows, there are usually more people performing on stage than there are in the audience. On this day, the lead actress, Rita Monroe, has just injured her ankle, leaving the assistant stage manager, a shy waif of a woman named Polly Browne to assume the lead female role just before her first scheduled entrance. The timing is bad as in the audience for this matinée is famed Hollywood movie director De Thrill, who is scouting for his new all talking movie musical. So there is much pressure on Polly not just to perform adequately, but to save the production to ensure it pleases De Thrill. While many of the actors end up playing solely to De Thrill - especially featured chorus members Maisie and Tommy, who try to one up the other on stage - Polly only wants not to embarrass herself, and have real life mirror the on-stage story as she is in love with the lead actor, Tony Brockhurst.