When Peter Lorre arrived in Great Britain, his first meeting with a British director was with Alfred Hitchcock. By smiling and laughing as Hitchcock talked, the director was unaware that Lorre, a Hungarian, had a limited command of the English language. Hitchcock subsequently decided to cast Lorre in this movie, and the young actor learned much of his part phonetically.

The dentist scene was originally intended to take place in a barber shop. However, Alfred Hitchcock saw I Am a Fugitive from a Chain Gang (1932), in which there is a scene exactly like it, so he changed it to a dentist's office.

Producer C.M. Woolf hated this movie and only allowed it to be released as the bottom half of a double bill. Nevertheless, it won rave reviews.

The crucial cantata for the Albert Hall sequence was composed specifically for this movie by Arthur Benjamin, and the same piece was used again in The Man Who Knew Too Much (1956). When Alfred Hitchcock remade this movie, he offered composer Bernard Herrmann the opportunity to compose a new work for the scene, but Herrmann chose not to, citing an appreciation of Benjamin's original cantata.

Peter Lorre's first English-language movie.

The script was originally written to be a Bulldog Drummond movie, titled "Bulldog Drummond's Baby," but Alfred Hitchcock wasn't able to get the rights to the "Bulldog Drummond" name.

Apart from the opening and end credits, there is only source music in this movie (for example, music that can be heard by the characters), such as dance music in Switzerland and Wapping and the Benjamin cantata (with the rest of the concert on the radio). There is no underscoring music at all.

The logo for Gaumont British Pictures is located on a scarf worn by Leslie Banks during the opening scene.

The title of this movie comes from the name of a book written by G.K. Chesterton.

The failure of the original copyright holder to renew the copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS or DVD copy of this movie. Therefore, many versions of this movie 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 movie.

Alfred Hitchcock reverted back to the more familiar territory of a suspense thriller after the disastrous performance of Strauss' Great Waltz (1934).

This film is part of the Criterion Collection, spine #643.

Alfred Hitchcock: (At around 36 mins) Just after the dentist office scene on the lamp post corner, entering stage right in a raincoat as a bus passes.

The siege at the end of the movie showing the police in a gun battle is based on the real-life Sidney Street siege, which took place on January 3, 1911, in which London police and a unit of the army's Scots Guards shot it out with a gang of heavily armed anarchists who had previously been involved in the murders of three police officers. Two anarchists and a police officer were killed.

The shootout was inspired by an actual event known as "The Sydney Street Siege" of 1910. 3 anarchists holed up in a flat had a protracted shootout with police.