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) literature. Her assigned Open University professor Frank Bryant (Michael Caine) however, openly abuses alcohol and soon develops misgivings about Rita's ability to adapt to student culture. Bryant is a jaded university lecturer, who describes his occupational ability as "appalling but good enough for his appalling students". His passion for literature is reignited by Rita, whose technical ability for the subject is limited by her lack of education but whose enthusiasm Frank finds refreshing. To illustrate the rhyming principle of assonance, the repetition of vowel sounds, Dr. Bryant gives as an example the words 'swans' and 'stones' from W B Yeats's