Trivia

At the request of director Billy Wilder, Miklós Rózsa adapted music from his Violin Concerto as the basis for the film score, supplementing this with further original music.
--Source: Nobody's Perfect: Billy Wilder, A Personal Biography, Page 256

With a 260-page script and a budget of $10 million, this was set to be a 165-minute Road Show picture with an intermission for comfort. The film was originally structured as a series of very specifically structured linked episodes, each with a particular title and theme: The dreadful business of the naked honeymooners, The curious case of the upside down room, The singular affair of the Russian Ballerina and The adventure of the dumbfounded detective.
--Source: A foreign affair: Billy Wilder's American films, Page 159

Billy Wilder: “I should have been more daring. I wanted to have Holmes homosexual and not admitting it to anyone, including maybe himself. The burden of keeping it secret was the reason he took dope.”
--Source: A foreign affair: Billy Wilder's American films, Page 147

Last cinema film of Catherine Lacey.

Stills and soundtrack show John Williams in a substantial role as a bank official and Edward Fox as Inspector Lestrade. All their scenes were ultimately cut.

Christopher Lee comes the role of Mycroft with considerable experience in the Sherlock Holmes universe: he previously played Sherlock Holmes and Sir Henry Baskerville.

Sir Robert Stephens and Sir Christopher Lee have both appeared in adaptations of The Lord of the Rings. Stephens played Aragorn in the BBC Radio broadcast, while Lee played Saruman in Peter Jackson's films.

Christopher Lee replaced George Sanders on short notice.

According to Billy Wilder, since because of schedule conflicts he couldn't himself supervise the bowdlerization of the picture demanded by the Studios, he entrusted the task to the editor, Ernest Walter. Nevertheless, Wilder supposedly strongly disliked the cuts made by Walter, and couldn't re-edit the movie because all the deleted scenes were lost or thrown away. Some of those scenes are available today, but never with both the audio and the video intact.

Originally Peter O'Toole was going to play Sherlock Holmes with Peter Sellers playing Dr. Watson, but Billy Wilder decided to go with lesser known stars instead.

Originally, the scenes featuring the Loch Ness Monster were intended to be filmed in the actual Loch. A life-size prop was built which had several Nessie-like humps used to disguise floatation devices. The humps were removed, however, at Billy Wilder's request. Unfortunately, during a test run in Loch Ness, the Monster-prop sank and was never recovered. A second prop (just the head and neck) was built, but was only filmed inside a studio tank.