Humphrey Bogart was involved in a serious automobile accident during production of this film, which knocked out several of his teeth and hindered his ability to speak. John Huston hired a young British actor noted for his mimicry skills to rerecord some of Bogart's spoken lines during post-production looping. Although it is undetectable when viewing the film today, it is Peter Sellers who provides Bogart's voice during some of the scenes in this movie.
John Huston was star/producer Humphrey Bogart's first choice to direct. However, Huston had some scheduling conflicts - he was due to make a movie with Katharine Hepburn (which was never made, as Hepburn graciously stepped aside to help out Huston), not to mention that he had to finish his then-current project Moulin Rouge. Nicholas Ray, who Bogart had worked with twice before, was considered to direct in case Huston could not finish in time.
Stephen Sondheim got his start in films working as a clapper boy on this film.
William Styron's second novel, "Set This House on Fire", describes a film crew on location - obviously based on director John Huston and gang during the shooting of this film. The town in the novel is Ravello on Italy's Amalfi Drive, where most of the film was shot.
At one point in the film, Ivor Barnard's character is referred as the "galloping major". This is the title of a film from 1951, also made by Romulus Productions, and starring Basil Radford. The Galloping Major in this other film is a racehorse.
Claud Cockburn (aka James Helvick) wrote the screenplay based on his novel. Truman Capote was purportedly brought to complete the script late in the piece when Cockburn left or was fired.
Either the writer or the director was playing an inside joke by naming two of the characters 'Chelm'. Chelm, in Yiddish folklore, refers to a village in eastern Europe that is ruled by the insane.
Jack Clayton, the Scotland Yard inspector played by Bernard Lee, is named after the film's cameraman, Jack Clayton, who later became a well-known director.
This was the fifth and last movie that Humphrey Bogart would make with Peter Lorre. The other four were, The Maltese Falcon, Casablanca, All Through the Night, and Passage to Marseille.