A German version called Mary was filmed at the same time using German actors, but the same sets.
The first film of Esme Percy.
The scene where Sir John thinks out loud in front of a mirror had to be filmed with a recording of the lines and an orchestra hidden behind the set as it was not possible to dub the soundtrack later.
This is the first movie where a person's thoughts are presented on the soundtrack of the film.
Alfred Hitchcock:
about an hour into the movie walking past the house where the murder was committed.