Much of the film was originally shot silent; when sound became available during the course of shooting, director Alfred Hitchcock re-shot certain scenes with sound, thus making it the Master of Suspense's first talkie. There was one complication with this change, however. Leading lady Anny Ondra had a thick Czech accent which was inappropriate to her character, Alice White. Joan Barry was chosen to provide a different voice for her, but post-production dubbing technology did not exist then. The solution was for Barry to stand just out of shot and read Alice's lines into a microphone as Ondry mouthed them in front of the camera. This is generally acknowledged as the first instance of one actor's voice being dubbed by another, even though the word "dub" is technologically inappropriate in this case.
In one key shot, the villain Cyril Ritchard is photographed with a thick shadow (caused by the arm of an overhead chandelier) across his upper lip. Hitchcock wanted the image to evoke the old-fashioned, heavily mustached villain found in many silent films. He later called this touch "my farewell to silent pictures".
With The Jazz Singer (1927) doing spectacular business, the producers decided that the last reel of this predominantly silent film should have sound. Alfred Hitchcock thought that this was an absurd idea and so he secretly filmed the whole thing with sound.
Generally acknowledged as the first British talkie, and definitely the first sound-on-film British talkie. The Clue of the New Pin (1929), was released in March 1929 in the British Phototone system, a sound-on-disc system using 12-inch phonograph records synchronized with the film.
There is a big mess with the title of this movie in Spain. The film was apparently not released there until the 70s, due to its status as one of the first sound movies. Then someone wrote a book which referred to this film by the wrong title, "La muchacha de Londres", which had actually been given to a different film starring Anny Ondra, Eileen of the Trees (1928). Thereafter this error was copied in various publications, and when the film was dubbed and aired on TV in 1984, this error was maintained. Meanwhile in the 70s, the film was first released in theaters in Spain, entitled "Chantaje", the title that it ought to have had in the first place, instead of "La muchacha de Londres".
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.
The trivia item below may give away important plot points.
The light levels in the British Museum were insufficient to allow Hitchcock to film the final chase scene in the museum. Without informing the producer, Alfred Hitchcock used the Schufftan process (developed by German cinematographer Eugen Schüfftan). This involved taking still photos of the interior of the museum, then reflecting the photos in a mirror with certain parts of the silvering of the mirror scraped away to allow people (entering a door, for example) to be filmed through the mirror so that they appeared to be present in the museum (in later years, American development of traveling matte and other process photography methods largely replaced the Shufftan process).