Peter Lorre was Jewish and fled Germany in fear of Nazi persecution shortly after the movie's release. Fritz Lang, who was half Jewish, fled two years later.
Contrary to popular belief, Fritz Lang did not change the title from "The Murderers are Among Us" to "M" due to fear of persecution by the Nazis. He changed the title during filming, influenced by the scene where one of the criminals writes the letter on his hand. Lang thought "M" was a more interesting title.
Fritz Lang asserts that he cast real criminals for the court scene in the end. According to biographer Paul Jensen, twenty-four cast members were arrested during filming.
Fritz Lang's cruelty to his actors was legendary here. Peter Lorre was thrown down the stairs into the cellar over a dozen times. When Lang wanted to hire Lorre for "Human Desire" over two decades later, the actor refused.
Based on an article Fritz Lang read about the serial killer Peter Kuerten from Duesseldorf. Details have been changed but some things resemble reality.
MGM studio head Irving Thalberg assembled his writers and directors for a private screening of this film, telling them that they needed to be making films of this power and caliber. He also admitted that if anyone had brought a story of a child killer to him, he would have rejected it.
Director Fritz Lang made this film in an effort to claw back his artistic standing after the double failure of his two previous films, Metropolis and By Rocket to the Moon.
Two German serial killers are mentioned in the film - Georg Karl Großman (believed to have killed up to 50 young women) and Fritz Haarmann (known as the Butcher of Hannover; killed at least 24 young men in Hannover).
M is supposedly based on the real-life case of serial killer Peter Kürten, the "Vampire of Düsseldorf", whose crimes took place in the 1920s, but Fritz Lang expressly denied that he drew any inspiration from the case.
Peter Lorre's character is introduced by the musical cue 'In the Hall of the Mountain King' from Edvard Grieg's Peer Gynt Suite No. 1. This was one of the very first times that a musical theme was used to signify a character - a technique borrowed from the world of opera which is now a staple of film-making.
The film has a very sour vision of contemporary life in Germany. This is probably due to the fact that Fritz Lang - a Jew - was alarmed at the rapid rise of Nazism and that even his wife Thea von Harbou had become a party member.
The film was independently backed by an admirer of Fritz Lang who persuaded him to make another film when the director was thinking of giving it all up. Lang eventually agreed to make the film provided that he had no interference and had final cut.
It was common practice at the time for foreign language films to be concurrently shot in English too. Fritz Lang had nothing to do with the English language version of his film.
The film premiered in 1931 and was then banned in 1934. It was then stuck in a vault for many years. Audiences didn't get the chance to see the film again until 1966. For its video release 30 years later, it underwent a restoration which included the addition of music and sound effects that wouldn't have been authorized by Fritz Lang (he deliberately kept certain passages quiet) and the cutting of certain scenes. The image had also been altered to fit the 4:3 screen size. These injustices were amended in 2009 for the film's Blu-ray release.
Reputedly based on Peter Kurten, the Vampire of Dusseldorf, a serial killer whose crimes horrified Germany in between the wars. Although Fritz Lang always denied that Kurten was the subject, nevertheless he and his wife Thea von Harbou researched the crimes carefully, consulting with the German police force, visiting murder scenes, interviewing sex offenders in prison and even talking to detectives in Scotland Yard in London. According to Lang biographer Paul Jensen, the director spent eight days doing field research in a mental institution.
Two thirds of the film was shot with sound, the remaining third was shot silent. At the time the license fees for sound equipment were quite prohibitive so this was a move to try to keep costs down. However, Fritz Lang quite liked the eerie, unnerving quality that arose from going from a sound world to one where there is no noise at all.
Although he was thrilled to play such a major part, Peter Lorre came to hate it later as people tended to associate him with being a child murderer in real life.