When Peter Lorre arrived in Great Britain, his first meeting with a British director was with Sir 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 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 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 Sir 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.
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 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.