"The Understudy" is the fifth episode of British dark comedy anthology series Inside No. 9. It was first broadcast on 5 March 2014 on BBC Two. The episode was written by and starred Steve Pemberton and Reece Shearsmith, and guest-starred Lyndsey Marshal, Julia Davis, Rosie Cavaliero, Roger Sloman, Di Botcher, Richard Cordery, Bruce Mackinnon and Jo Stone-Fewings. Pemberton plays actor Tony, who is starring as Macbeth in a West End production of Shakespeare's Macbeth, and Shearsmith plays Jim, Tony's understudy. The plot of "The Understudy" partially mirrors the story of Macbeth, exploring the theme of power and the lives of actors.

Inside No. 9 is a British black comedy anthology television programme that first aired on 5 February 2014. It is written by Steve Pemberton and Reece Shearsmith and produced by the BBC. Each 30-minute episode is a self-contained story with new characters and a new setting, almost all starring Pemberton or Shearsmith. Aside from the writers, each episode has a new cast, allowing Inside No. 9 to attract a number of well-known actors. The stories are linked only by the number 9 in some way, typically taking the form of a door marked with the number 9, and a brass hare statue that is in the background of all episodes. Themes and tone vary from episode to episode, but all have elements of comedy and horror or perverse humour, in addition to a plot twist. Pemberton and Shearsmith took inspiration for Inside No. 9 from an episode of Psychoville, a previous project, which was filmed in a single room – this in turn was inspired by Alfred Hitchcock's Rope.