The clock tower scene is the first major use of computer animation (the clock's gears) in a feature-length animated film. The same scene was also the first time traditionally-animated characters were put inside a computer-generated background.
The "Let Me Be Good To You" segment was almost cut because though brief, the lyrics and some animation was considered "too risqué" for a Disney animated family film, the animators avoided a PG rating and got the scene kept in by appealing to the censors on the grounds that the segment was a Caberet song and harmless in lyrics, and because the character animated singing it was a mouse, not a human and thus not questionable.
Basil of Baker Street is named after Basil Rathbone, who played Sherlock Holmes in 14 films. By strange coincidence, Basil was also a name used by a disguised Holmes in Arthur Conan Doyle's "The Adventure of Black Peter."
As Ratigan is ranting about how much he hates Basil, we see that he has a voodoo doll in the shape of a mouse in a deerstalker. The doll bears a striking resemblance to Basil, not as he appears in this film, but as he appears in Paul Galdone's illustrations in the book on which the film is based.
Alan Young had performed a near-perfect Scottish accent as the voice Scrooge McDuck for the 1977 Disneyland Records adaptation of "A Christmas Carol" (which he also developed and wrote). He repeated the role of McDuck in Mickey's Christmas Carol (1983), and was a natural for the Scotish brogue of Hiram Flaversham.
It was ultimately Vice President of Walt Disney Feature Animation Peter Schneider who made the decision to change the title of the film from "Basil of Baker Street" to its current title. On February 13, 1986, an inter-office memo was sent out to Disney employees in Schneider's name announcing the renaming of the studio's most beloved classics. Snow White and the Seven Dwarfs (1937) would be called "Seven Little Men Help A Girl", Fantasia (1940) received the title "Color And Music", The Jungle Book (1967) was getting its title changed to "A Boy, A Bear And A Big Black Cat" and so on in that fashion. Schneider was furious over the memo and attempted to find the author (animator Ed Gombert) so he could fire them. All the other employees found it a harmless joke and kept quiet. A copy of the memo eventually landed in the pages of the LA Times and all the "new" names were incorporated into the "What's In A Name?" category on Jeopardy! (1984).
When designing Fidget, the Disney animators were focused on creating a scary, yet comical and lovable character. They were looking for a raspy voice and chose Candy Candido, who had starred in voice-over roles in many previous Disney films as well as Ralph Bakshi films. His own looks were used in matching Fidget's looks. Candido's deep, throaty voice was sped up to avoid Fidget's voice from becoming too low. (Candido's original voice can be heard as the mouse shouting "Get off, you eight-legged bum!" at the juggling octopus in the pub.)
Sherlock Holmes speaks with the voice of Basil Rathbone. Although it is often erroneously claimed that the lines are taken from one of Rathbone's 1940s performances as Sherlock Holmes on film or radio, this is not true. The cameo is edited from Rathbone's reading of the Sherlock Holmes story "The Adventure of the Red-Headed League" by Arthur Conan Doyle for Caedmon Records in 1966, just months before his death. This explains why Rathbone's voice sounds older and less crisp than in his famous films, and more importantly, why the voice of Rathbone's co-star Nigel Bruce was not used for Dr. Watson's brief cameo. According to the text of "The Adventure of the Red-Headed League", this would mean that the film takes place sometime in the autumn of 1890.
The first meeting between Basil and Dawson where Basil guesses he came from Afganistan and gives a complicated explanation how he assume it, mirrors first meeting between Holmes and Watson in Arthur Conan Doyle's "A Study in Scarlet" (the first Holmes novel) where Holmes does the same thing.
CASTLE THUNDER: It is heard every time lightning flashes during the storms in this film, and a version played at slow speed is heard a few times. This was the last animated Disney movie to regularly use the Castle Thunder sound effect. Starting with The Black Cauldron (1985) the previous year, Disney began trying out newer, digitally-recorded thunder sounds.
"Let Me Be Good To You" was originally to be sung by Madonna, but the directors decided that this was not contemporary enough for the audience to enjoy. After Liza Minnelli was briefly considered, Melissa Manchester was hired as the new singer of the song. "Let Me Be Good To You" was also once entitled, "Look At Me".
According to the newspaper ("The Illustrated London Mouse," number 1234) with the article entitled, "Queen Honours Detective," the film ends on (or very close to) Monday, June 21, 1897. The article can be read (although difficult to make out); the first column starts out with, "The (...) Prince and Princess of Germany were present on Monday at a jubilee demonstration of the (...) children of (...)," the second column begins, " The Queen, accompanied by Prince and Princess Henry of (...) and Princess (...) of (...), left (...) on the afternoon of Thursday (...)," and the third column starts out with, "We are (...) to state that the number of (...) addressed to the Queen conveying kind and loyal (...) to her Majesty on the (...) back from public (...) and (...) individuals is an (...) that it has (...)"
When this film was originally released its title was "The Great Mouse Detective." When Disney re-released it years later they gave it the title of "The Adventures of the Great Mouse Detective." When the film was released on video a few months later, the title on the box was back to "The Great Mouse Detective" but the title on the film itself read "The Adventures of the Great Mouse Detective."
The trivia items below may give away important plot points.
As Basil was based on Arthur Conan Doyle's Sherlock Holmes, Ratigan was based on Holmes' arch-nemesis Moriarty, and, like Moriarty, Ratigan fell to his death with Basil (Holmes) while Basil escaped death.