Luis Buñuel wrote in his autobiography ("My Last Sigh"), he was employed by Warner Bros. and submitted a story idea for a horror movie about a disembodied hand.
According to Curt Siodmak in 'Screenwriter: Words Become Pictures' he originally wrote the story for Warner Brothers leading man 'Paul Henried'. The actor declined saying, "I'm not wild to play against a dead hand." The writer believed that it would have been more effective with a good-looking man like Henried instead of Lorre, whom the audience would automatically think was crazy.
The piece of piano music played by Francis Ingram (Victor Francen) and later, his disembodied left hand, is the Bach Chaconne in D minor, as arranged to be played by the left hand alone by Johannes Brahms. It was selected by Max Steiner because the story required a piece of music that could be performed by a pianist with only his left hand, and Steiner, who was born in Vienna and whose family were friendly with Brahms, rather than composing his own original piece, immediately recognized its potential in underscoring such a grim tale. Legendary Hungarian-American pianist Ervin Nyíregyházi performed the music played by the severed hand.
This was Peter Lorre's last film for Warner Bros.
Though called "Bruce Conrad" in the script, Robert Alda is billed as "Conrad Ryler"; Charles Dingle is billed as "Raymond Arlington"; Ray Walker is billed as "Mr. Miller".