Not only did all the actors do their own singing, but everyone in the cast, including the pit orchestra and the actors who play instruments in the film, actually played the music they are seen to play.
Most modern recordings and performances of the Mikado's solo, "A More Humane Mikado" feature a bloodthirsty laugh between the verses. This touch was added by Darrell Fancourt, a D'Oyly Carte performer from 1920-1953, and has been copied ever since - which is why the laugh is not performed by Richard Temple.
Mike Leigh was so frustrated with the continued relegation of his movies to art house cinemas instead of wide release that he joked that he would have cast Arnold Schwarzenegger as both Gilbert and Sullivan if his budget had permitted it.
Though the film shows Gilbert coming up with the idea for The Mikado after visiting the Japanese exhibition, in reality he had already finished writing the first act of the libretto by the time the exhibition came to London.
Much of the six-month-long rehearsal process consisted of the cast discussing and doing detailed research on Victorian social and personal habits, speaking patterns, news events, and the mechanics of theatrical production during that era.