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
Rita Lakin wrote or co-authored two original theatrical plays, "No Language But a Cry" and "Saturday Night at Grossinger's". Rita Lakin has won nominations and awards for her writing including the Writers Guild of America Award, the Edgar Allan Poe Award and the Avery Hopwood Award from the University of Michigan. Rita Lakin has written 464 episodes, eight movies of the week, and two miniseries, spanning 30 television productions. She is also the creator of the crime drama series "The Rookies" and the prime time soap "Flamingo Road", and co-creator with Howard Lakin and Frank Furino of the US medical drama "Nightingales"