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 (film) the latter. No. 8 Hogan Avenue in Dublin 2 near Grand Canal Dock was used for Rita's house in the film, and one in Burlington Road, Ballsbridge for Bryant's. The scene where Rita runs into her ex Denny and his new wife was filmed in the South Lotts area of Ringsend. The scene in France was filmed in Maynooth, County Kildare, and Pearse Station and Dublin Airport were also used. The scene in the pub was shot in The Stag's Head pub on Dame Court in Dublin. However, the pub which Rita enters is the Dame Tavern which is opposite