When shooting the scene in which Salieri is writing down the death mass under Mozart's dictation, Tom Hulce was deliberately skipping lines to confuse F. Murray Abraham in order to achieve the impression that Salieri wasn't able to fully understand the music being dictated.
The film ironically helped spark a revival of Salieri's music, which had previously languished in obscurity.
Tom Hulce only knew how to play the guitar before shooting. Milos Forman said they could cheat it, but it would be good if he learned how to play the piano. Hulce spent six hours a day for six months learning how to play the piano, and every Mozart symphony that was in the film.
Several professors of music stated, after studying all of the musical keys struck on pianos throughout the film, that not one key is struck incorrectly when compared to what is heard at the exact same moment. In other words, what you see is exactly what you hear.
Milos Forman insisted that his lead actors retain their American accents so that they could concentrate on their characters and performances instead.
Milos Forman: [Schiavelli] Vincent Schiavelli appeared in almost every Forman film. Here he's Salieri's valet.