Peter O'Toole was originally cast as Sherlock Holmes, and Sir Laurence Olivier was cast as Dr. Watson. The two actors had not worked well together in the past, and were unable to overcome their differences for this movie.
One of the last actors cast was James Mason. Director Bob Clark had to travel to Spain, where Mason was shooting another movie, to meet with him. Mason agreed to play Watson if he could make him a serious character, not the "silly ass" buffoon which was the rule. Mason re-wrote two scenes, including the famous "pea scene".
Sherlock Holmes is a fictional character, and Sir Arthur Conan Doyle never wrote a story about Jack the Ripper, but the idea of Holmes working on that case has some basis in fact. Scotland Yard consulted Joseph Bell, who inspired the character of Sherlock Holmes, about the case.
Christopher Plummer is a second cousin of Nigel Bruce, who played Dr. Watson in several previous "Sherlock Holmes" films.