In his autobiography Callow states that he read in Screen International that both he and Ian McKellan had been cast in the film of Amadeus. However, he says that no one spoke to either of their agents or approached them, so these were just rumors. He said there were also rumors that Dudley Moore would play Mozart and Liza Minelli would be Constanza.
After a number of actors had tested, Callow was called to meet Forman who had seen him play Mozart onstage. Forman said Callow was the best Mozart he had seen. He mused about what role he could cast Callow in for the film but could not think of one so the meeting ended.
Days later Forman asked him to test for Mozart. He did. Heard nothing. Then read in the papers that Hulce had been cast.
Later they offered him the role of the Magic Flute librettist, which is the role he had in the film.