Sir Ian McKellen took a course in beekeeping with The London Honey Company prior to filming. He was not stung during filming.

Sir Ian McKellen didn't research Sherlock Holmes stories in preparation for his role. McKellen claimed that making The Lord of the Rings trilogy taught him that knowing the source material, and constantly questioning the changes were made for the script, was nothing more than a distraction.

Sir Ian McKellen observed that it was easy for him to imagine Sherlock Holmes as a real person. The majority of this movie is set during his own lifetime, and he could have met Sherlock as a boy in 1947, just like Roger.

To simulate age progression, liver spots are placed on Holmes' head. In earlier scenes, they are not there.

This movie originally was titled "A Slight Trick of the Mind", after the Mitch Cullin novel on which it was based.