For the opening of the film, Alfred Hitchcock wanted to show the Avenger's murder victim being dragged out of the Thames River at night with the Charing Cross Bridge in the background, but Scotland Yard refused his request to film at the bridge. Hitchcock repeated his request several times, until Scotland Yard notified him that they would "look the other way" if he could do the filming in one night. Hitchcock quickly sent his cameras and actors out to Charing Cross Bridge to film the scene, but when the rushes came back from the developers, the scene at the bridge was nowhere to be found. Hitchcock and his assistants searched through the prints, but could not find it. Finally, Hitchcock discovered that his cameraman had forgotten to put the lens on the camera before filming the night scene.
For the opening scene, where the Avenger's murder victim faces the camera and screams, Alfred Hitchcock filmed the scene by having the actress lie down on a large sheet of glass, with her golden hair spread out around her head. He then lit the actress from underneath the sheet of glass, and filmed her with a camera mounted on its side, with the lens pointed at a downward angle. This gave the appearance that the actress's hair (with its golden curls, so important to the murderer) was ringed in a halo of light.
Alfred Hitchcock's cameo as an extra came by accident when he didn't have enough people for extras in a scene, he decided to help by appearing in the scene himself. As a result, he decided to turn his appearance into one of his trademarks with him performing silent walk-on bits in most of his later films appearing as uncredited extras.
The book "The Lodger," by Marie Belloc Lowndes, was the first book to offer a solution to the Jack The Ripper killings. The book is supposedly based on an anecdote told to the painter Walter Sickert by the landlady when renting a room; she said that the previous tenant had been Jack the Ripper. The book was quite popular in its day, was filmed numerous times, and adapted for the radio multiple times, once with Peter Lorre as the lodger.
Producer Michael Balcon was horrified by Alfred Hitchcock's progressive style of filming, not to mention the implications of homosexuality and incest. He called in critic Ivor Montagu to trim the film. One of Montagu's chief acts was to reduce the number of title cards from 400 to 80.
Arthur Chesney's elder brother Edmund Gwenn would later succeed him in the role of Mr. Bunting in the CBS radio adaptation of "The Lodger" on July 22, 1940, which was likewise directed by Alfred Hitchcock. That episode served as the pilot episode of the long-running radio series "Suspense" which began two years later.
The trivia item below may give away important plot points.
The original novel ends ambiguously, with the reader never sure if the lodger was the murderer or not. Reportedly, Alfred Hitchcock wanted to film it that way, but the studio forced a rewrite of the ending, as it was felt that audiences wouldn't like a popular star like Ivor Novello to be shown as a possible killer. Some other adaptations of the story have also changed the ending, to one where the lodger is definitively proven to be the killer.