Much of the filming originally was shot silently. When sound became available during the course of shooting, Sir Alfred Hitchcock reshot certain scenes with sound, thus making it his first talkie. There was one complication with this change, however. Leading lady Anny Ondra had a thick Czech accent which was inappropriate for 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 Ondra mouthed them in front of the camera. [This is a major plot point of Cantando na Chuva (1952), which is set in the era of movie studios moving from silent pictures to talkies.] This generally is acknowledged as the first instance of one actress' voice being dubbed by another, even though the word "dub" is technologically inappropriate in this case.
In one key shot, Mr. Crewe, the artist, (Cyril Ritchard) is photographed with a thick shadow (caused by the arm of an overhead chandelier) across his upper lip. Sir Alfred Hitchcock wanted the image to evoke the old-fashioned, heavily mustached villain found in many silent movies. He later called this touch "my farewell to silent pictures".
The light levels in the British Museum were insufficient to allow Sir Alfred Hitchcock to film the final chase scene in it. Without informing producer John Maxwell, 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 travelling matte and other process photography methods largely replaced the Shufftan process.)
When this movie was released, the silent version did considerably better business than the sound one, as few theaters outside of the big cities were equipped for sound.
DIRECTOR_CAMEO(Alfred Hitchcock): A signature occurrence in many of Hitchcock's films, his cameo shows him being bothered by a small boy as he reads a book on the London Underground. This is probably the lengthiest of Hitchcock's cameo appearances and he appears around at 13:40 minutes after the start. As the director became better known to audiences, especially when he appeared as the host of his own TV series, he dramatically shortened his on-screen appearances.