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
Educating Rita relationships. The play borrows from the George Bernard Shaw play "Pygmalion", itself based upon archetypes from Greek myth. The play was adapted by Russell for a 1983 film with Michael Caine and Julie Walters, directed by Lewis Gilbert. The play was adapted by Russell for radio in 2009 – see "Educating Rita (radio play)". It starred Bill Nighy and Laura Dos Santos directed by Kirsty Williams, and was a 90-minute play broadcast on BBC Radio 4 on Boxing Day 2009. From June–July 2001, the Williamstown Theatre Festival mounted a production of Educating Rita at the Nikos Stage starring Jacqueline McKenzie