The film ends with a long take of Eddie sitting in his room with a hat on, smoking a cigarette and listening to a record. Writer Neville Smith wanted the record to be an authentic rock'n'roll classic, perhaps Elvis Presley's original recording of "Blue Suede Shoes", but the rights to this and other recordings of the period were prohibitively expensive and it was cheaper for Andrew Lloyd Webber and Tim Rice to write a new song instead.
Publicity for this picture explained the meaning of the film's title by quoting its definition from the American Dictionary of Slang which defined a GUMSHOE as "a hard, foot-slogging private eye".
Albert Finney was originally going to direct this film, having recently directed "L'errore di vivere (1968)," but he was so immersed in his characterization for the film that he decided to forego directing the picture, and handed over the reins to his "L'errore di vivere (1968)" assistant Stephen Frears.
The second film produced by Albert Finney's production company "Memorial Enterprises." Finney, uncredited, was a producer on the picture. The first production was the previous year's "Spring and Port Wine" starring James Mason. Producer, Michael Medwin, also makes a very brief, uncredited cameo in it.