Covered Up: The theme song "Original Sin" was originally the title track for a Pandora's Box album written and produced by Jim Steinman and was later more famously covered by his frequent collaborator Meat Loaf (with some lyrical alterations: only the movie version features lines about "who knows what evil lurks in the hearts of men").
The shooting script had a longer chase scene in the Hall of Mirrors, including flashbacks and banter. However, an earthquake destroyed some of the mirrors, making the full scene too expensive.
Sam Raimi wanted to follow up Evil Dead 2 with this but was unable to get the rights. Instead, he directed Darkman, using some of the ideas he had for his version of the story.
The Other Darrin: Tim Kitzrow provides the voice for Lamont Cranston/The Shadow, though Baldwin's speech from the film itself appears as well.
Pulps and Radio
Actor Allusion: Possibly. In his later years, Orson Welles's outfit of choice when appearing in public was loose-fitting black, oftentimes with a cloak and matching fedora. No red scarf, though. In The Third Man he wore a black coat and fedora like The Shadow.
Amusingly, The Shadow as depicted on radio never wore a costume.
Defictionalization: The Shadow actually began as just the host character of a radio adaptation of the Detective Story magazine. When people kept asking for his non-existent magazine, Street and Smith ended up creating one with Walter Gibson as the main writer.
Executive Meddling: Worried that the character was getting too powerful and too difficult to challenge, the writers were ordered to scale back the character's powers to just invisibility (and that they add in weaknesses to even that), and restricting Cranston to using invisibility only twice an episode (at the halfway mark and right at the end).
The original Margo Lane was played by Agnes Moorhead, who later played Endora on Bewitched.
The original voice actor for cab driver Moe "Shrevie" Shrevnitz was Alan Reed, otherwise known as the voice of Fred Flintstone. Reed also often played other Large Ham characters, especially villains.
After Welles left the show, Bill Johnstone took over the role. While not very well-known today, he was a prolific radio actor and showed up in a lot of shows of the time, most regularly as Dr. Franz, sidekick and mentor to the radio Blue Beetle, and he even had a major role in the very first Orson Welles episode of the Shadow (as a man falsely accused of murder) and turned up in other roles on the show even after he left the lead role.