Acting for Two: Barrie Ingham voices both Basil and the unlucky Bartholomew. Interestingly, that makes two out of the three characters in the movie who call Ratigan a rat (the other being Olivia), and one out of two that live to tell the tale.
Actor Allusion: When Bill the Lizard tries to assure Ratigan that he's a "big mouse" to convince him to spare Bartholomew, his voice suddenly switches to a high-pitched falsetto, referring to the particular big mouse his voice actor was best known for providing.
Awesome, Dear Boy: Price on his role as Ratigan. He had stated that Ratigan was his favorite film role.
Having two songs written for him certainly helped.
In the Mexican Spanish dub, Ratigan was played by popular Mexican actor Narciso Busquets.
Likewise in the Japanese dub, where Ratigan is voiced by veteran actor Akira Takarada.
Dueling Movies: With An American Tail, another animated movie about mice coming out the same year (though it didn't come out until a few months afterward). While the film made its money back, American Tail was a smash hit and, at the time, the highest-grossing animated feature ever made. Both of these things would factor into Disney deciding to re-beef their animation department.
Executive Meddling: Largely averted, as Jeffrey Katzenberg had little to no faith this movie would be any good. Most of his attention was spent on meddling with The Black Cauldron, which would become a failure. Katzenberg did, however, demand the title of this film be changed from "Basil of Baker Street" to simply "The Great Mouse Detective", a move that inspired an infamous fake memo that became a category on Jeopardy!. Two of the four directors have since said they've always hated the new title. However, those directors may have had some solace in the title being changed to "Basil the Great Mouse Detective" in some international versions, including Australia.
I Am Not Shazam: The lead dancer at the bar is listed in the credits as "Miss Kitty Mouse," but, Fanon aside, this doesn't actually appear to be her name. Rather, it's a description of her. She isn't "Miss Kitty Mouse," but "the Miss Kitty mouse."