Sir John thinking out loud in front of the mirror had to be filmed with a recording of the lines and a 30-piece orchestra hidden behind the set, as it was not possible to post-dub the soundtrack. For the filming, an orchestra played the music live on the set. Sir Alfred Hitchcock described the filming of this scene to François Truffaut in the book-length interview Hitchcock/Truffaut (New York: Simon & Schuster, 1967). In the early days of sound film, there was no way to post-dub sound, so Hitchcock had Sir Herbert Marshall's voice recorded on a phonograph record, which was played back during filming, while the orchestra played the "radio" music live.
This is the first film in which a person's thoughts are presented on the soundtrack.
The character of Sir John Menier (Sir Herbert Marshall) was loosely based on Sir Alfred Hitchcock's friend actor Gerald du Maurier. Hitchcock later directed three films adapted from novels by du Maurier's daughter Daphne Du Maurier: A Estalagem Maldita (1939), Rebecca, a Mulher Inesquecível (1940), and Os Pássaros (1963).
Because they distributed the film in the US, this is the closest Sir Alfred Hitchcock ever came to working with Columbia Pictures. (This film was a UK import.) He directed films for every other major studio in America but never directly for Columbia. That said, they did syndicate the two films Hitchcock made for Universal in the 1940s (Sabotador (1942) and A Sombra de uma Dúvida (1943)) under the name of Screen Gems, then used as a means for Columbia to produce/distribute shows and syndicate films from both Columbia and Universal (until 1962 for the latter, when Universal was acquired by MCA, then a major player in television syndication and production and where Alfred Hitchcock Apresenta (1955) was being distributed). Hitchcock would produce his remaining films at Universal.
Alfred Hitchcock: (at around 1h 2 mins) Walking past the house with a female companion where the murder was committed.