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 Valentine" and "Educating Rita" became successful feature films with Michael Caine, Julie Walters and Pauline Collins all winning Oscar nominations for their respective roles, as did Russell for his "Educating Rita" screenplay. Russell's other worldwide theatrical success has been "Blood Brothers", 'a Liverpudlian folk opera' about a pair of twins separated at birth and brought up in completely different environments. "Blood Brothers" won the Laurence Olivier Award for Best New Musical in 1983 and its 1987 revival saw over 10,000 consecutive performances during its 24-year West End run. Simultaneously, UK touring and international productions (including a two-year run on Broadway)