As was common in German movies of the era, this movie was shot without live sound, and all voices were dubbed in post-production. Sir Christopher Lee and Thorley Walters were not scheduled to dub their own voices for the English dubbed version (or any other), and were dubbed by other actors.
While shooting parts of this movie in Dublin, some gypsy extras came up to Sir Christopher Lee between takes and inquired about his origins. From the observation of his physical type and the few things Lee told them about his father, their conclusion was that he must definitely have been of some gypsy ancestry himself. Knowing very little about his father's family, Lee gladly welcomed this assumption as a very believable one.
When Sir Christopher Lee and Leon Askin were filming the scenes down the sewer seen near the end of the movie, they found the smell to be "a bit stiff, even for a sewer". After some inquiries, they found out the place had been used during World War II to make poison gas.
Holmes is seen taking tobacco for his pipe from his Persian slipper (as mentioned in the original stories "The Musgrave Ritual", "The Naval Treaty", and "The Adventure of the Empty House" ).
Sir Christopher Lee starred in at least six Sir Arthur Conan Doyle filmed adaptations or related filmed productions. They are: The Hound of the Baskervilles (1959), Sherlock Holmes and the Deadly Necklace (1962), The Private Life of Sherlock Holmes (1970), Sherlock Holmes and the Leading Lady (1991), Sherlock Holmes: Incident at Victoria Falls (1992), and Orson Welles' Great Mysteries: The Leather Funnel (1973).
Reportedly, Christopher Lee was disappointed not to have added his own voice onto the film soundtrack in post production.
This was the first of four times that Thorley Walters played Watson, each with a different actor playing Sherlock Holmes. He later did so in The Best House in London (1969) starring Peter Jeffrey, The Adventure of Sherlock Holmes' Smarter Brother (1975) starring Douglas Wilmer and The Sunday Drama: Silver Blaze (1977) starring Christopher Plummer.