Enforced Method Acting: While shooting the scene where a dying Mozart dictates the Requiem to Salieri, Tom Hulce deliberately skipped lines so as to make it seem Salieri could not understand what he was being dictated.
Mark Hamill was considered for the role of Mozart (for the film) and he had already played the role on Broadway. He was rejected because the director wanted lesser-known actors.
A little bit of a tangent, but interesting: When auditioning to voice the Joker, he simply reprised his Mozart laugh from when he played the composer on Broadway. It worked.
Kenneth Branagh, Tim Curry and Mel Gibson were among those rejected for the part of Mozart.
Meg Tilly was originally supposed to play Mozart's wife but she injured her leg the day before filming and the part had to be recast. Director Miloš Forman did later hire her again for a another period film set in the 18th century, Valmont (based on the novel Dangerous Liaisons).