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.

The plot of "The Understudy" partially mirrors the story of Macbeth, exploring the theme of power and the lives of actors.

They stressed the links between "The Understudy" and Macbeth, but said that viewers would not need to be familiar with Shakespeare's work in order to enjoy the episode.

Writers Steve Pemberton and Reece Shearsmith, who had previously worked together on The League of Gentlemen and Psychoville, took inspiration for Inside No.

9 was a "reaction" to Psychoville, with Shearsmith saying that "We'd been so involved with labyrinthine over-arcing, we thought it would be nice to do six different stories with a complete new house of people each week.

[1] In addition to Pemberton and Shearsmith, who played lead actor Tony and understudy Jim respectively, the episode starred Lyndsey Marshal as Laura, Julia Davis as Felicity, Rosie Cavaliero as Kirstie, Roger Sloman as Bill, Di Botcher as Jean, and Richard Cordery as Nick.

[5] The plot is essentially based on Macbeth, leading Pemberton to suggest that the episode might be used "on an O Level syllabus" in the future.

[6] Unlike other episodes in the series, "The Understudy" did not take place in a family home, and for this reason, the writers were keen not to alienate viewers.

Pemberton said that the West End setting allowed for the "agonising" scene of a post-performance visit from audience members.

Nineteen months later, Jim is a famous actor playing the title role in Richard III at the theatre, using the same, now redecorated, dressing room.

Critics responded very positively to "The Understudy", with several drawing attention to the improvement over "Last Gasp", the previous week's episode.

[8][9] Writers in The Sunday Times called the episode "[a]nother exquisite short story",[10] Ben Lawrence, writing in the Daily Telegraph, called it a "gloriously OTT tale",[11][12] and John Robinson, writing for The Guardian, described it as an "excellent instalment" of the series.

"Knowing the text", suggested Jack Seale of Radio Times, "will take you only halfway and, in any case, the clever plot is really just a vehicle for characters sketched fully in only a few lines, and a torrent of fruity luvvie gags about jealousy, superstition and stage-hogging hams.

[7] Critic Jane Simon, writing in the Daily Mirror, suggested that "even a hazy knowledge of the Scottish play will tip you the wink what might lie ahead".

[15] David Chater in The Times and an anonymous reviewer in the Sunday Herald both praised the plot's divergence from Shakespeare, with the latter saying that "the script cleverly offers parallels with The Scottish Play, just to whip them away again".

[16][17][18] Dessau felt that the complex plot meant that the guest stars did "not get quite as much screen time as they deserve", but that this was no complaint, as it was "very much Pemberton and Shearsmith's instalment and they are both brilliant".

[19] This was lower than "Last Gasp", the previous episode, which in turn had had the lowest viewership of the series on its first airing, with 872,000 viewers (4.9% of the audience).

[20] "The Understudy" was immediately preceded in most listings by Line of Duty, which was watched by a series high of 2.3 million viewers (9.9% of the audience).