Director Jules Dassin made the film while in the process of being blacklisted. Fox studio chief Darryl F. Zanuck told him it could possibly be the last film he'd ever direct, so he should shoot the most expensive scenes first so the studio wouldn't be able to blacklist him until it was completed.
Richard Widmark stated in an interview that he lost a great deal of weight during the production because of the all running he had to do.
According to Twentieth Century-Fox publicity at the time of the film's release, a total of fifty-four different London locations were used, and only fourteen interior sets were used.
Harry Fabian (Richard Widmark) is far more sympathetic in this movie than he was in Gerald Kersh's novel, in which Harry was not above pimping his girlfriend and blackmailing. Also, unlike the Harry Fabian in the film, he was a Brit masquerading as an American in order to impress others.
Twentieth Century-Fox studio head Darryl F. Zanuck chose this film to be set in London, so that the studio could use up some of the money frozen there by British laws enacted to protect that country's film industry at the time.
Kay Kendall: as one of Helen's girls, one on the right-hand side of the frame wearing a light cardigan and a string of pearls, just before Helen talks with a policeman in her nightclub at around the 86-minute mark in the movie.