- Peter Lorre's whistling was dubbed by director Fritz Lang.
- The first big German sound movie.
- The tune that Peter Lorre's character whistles is "In the Halls of the Mountain King," from the "Peer Gynt" suite, by Edvard Grieg.
- Fritz Lang and Peter Lorre were Jewish and fled Germany in fear of Nazi persecution shortly after the movie's release.
- Based on an article Fritz Lang read about the serial killer Peter Kuerten from Duesseldorf. Details have been changed (Kuerten was not a child murderer, for example) but some things resemble reality.
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