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 he learned his lesson the hard way, making The Lord of the Rings trilogy, that it was nothing more than a distraction, knowing the source material, and constantly questioning what changes were made for the script.
Sir Ian McKellen observed that it was easy for him to imagine Sherlock Holmes as a real person, since the majority of the film is set during his own lifetime, and he could have met Sherlock as a boy in 1947, just like Roger.
The film originally was titled "A Slight Trick of the Mind", after the Mitch Cullin novel, on which it was based, but the filmmakers changed it to the simpler, and more accessible "Mr. Holmes".
The house, in which Holmes lives, is on the edge of Winchelsea in Sussex, a medieval walled new town built after old Winchelsea slipped into the sea near Rye. Winchelsea is a walled town, about forty miles fr om Seven Sisters, wh ere the beach scenes were filmed.
Sir Ian McKellen stated in an interview, that his costume had very uncomfortable materials, and he chafed in the worst places. Because of this, he is hesitant about reprising his role in future instalments.