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
International Theatre Vienna Gin Game, The Skin of Our Teeth, The Matchmaker, The Removalists, The Collector, The Miracle Worker, On Golden Pond, Eden End, What the Butler Saw, Crimes of the Heart, The Foreigner, Baby with the Bathwater, Beyond Therapy, Parallel Lives, Educating Rita, Shirley Valentine, Charley's Aunt, Harvey, Arsenic and Old Lace, The Odd Couple, Barefoot in the Park, Moon Over Buffalo, Lend Me a Tenor, Play it Again Sam, The Mousetrap, Dial "M" for Murder, Deathtrap, Wait Until Dark, Rebecca, The Hound of the Baskervilles, Dracula, Night, Mother, Hamlet, Twelfth Night, Pygmalion, Arms and the Man, Into the Woods, Quilters, 'Art',