Fritz Lang's cruelty to his actors was legendary. Peter Lorre was thrown down the stairs into the cellar over a dozen times. When Lang wanted to hire Lorre for Human Desire (1954) over two decades later, the actor refused.
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 that is now a staple of filmmaking.
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.
MGM studio executive 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.
The film is supposedly based on the real-life case of serial killer Peter Kürten, called "The Vampire of Düsseldorf", whose crimes in the 1920s horrified Germany. However, director Fritz Lang has expressly denied that he drew any inspiration from the case. Nevertheless, he and his wife Thea von Harbou researched the crimes carefully, consulting with German police, 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.
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.
Peter Lorre couldn't whistle well so the whistling of "In the Hall of the Mountain King" heard in the film was actually done by Fritz Lang. Although the director had no musical talent, he felt his own off-key whistling very appropriate.
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).
It was common practice at the time for foreign-language films to be concurrently shot in English, for the British and North American markets. Fritz Lang had nothing to do with the English-language version of his film.
The film premiered in 1931. Adolf Hitler and his Nazi party took power in 1933, and banned the film the next year. It was then stored in a vault, where it stayed 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 that 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.
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 liked the eerie, unnerving quality that arose from going from a sound world to one where there is no noise at all.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.