The Riddle of the Sphinx (Inside No. 9) was characterised by Pemberton as a thriller in the style of Anthony Shaffer's 1970 play "Sleuth", while Shearsmith identified the episode as particularly theatrical, in the style of "Sleuth" and Ira Levin's 1978 play "Deathtrap". BBC executives, according to Shearsmith, saw the episode as a cross between "Sleuth" and Willy Russell's 1980 play "Educating Rita" that read as if it had been authored by psychopaths. The executive producer Jon Plowman suggested that the first half of the episode plays out like a radio play. The word-play and tea-drinking in this part of the episode are, in the writers' opinion, very
Willy Russell West End run, the play has been translated and produced in almost every part the globe garnering awards both for its author and for many of the actors who have played the roles of Rita and Frank. Returning to the Liverpool Everyman in 1986, Russell created "Shirley Valentine" which went on to an acclaimed West End run, earning Olivier awards for both its author and star, Pauline Collins, before transferring to New York for a highly successful Broadway run and a Tony Best Actress award for Collins. Like the earlier "Educating Rita", "Shirley Valentine" became an international hit. Both "Shirley