I've learned from multiple sources (IMDb, BuzzFeed) that every single note was played perfectly in Amadeus. My question is this: was the person playing those notes the actor Tom Hulce himself, or a double? Did he have training in music? Did he learn to play in order to prepare for the role?
It would appear that Hulce played the piano for the part, but this was so that his performance would be more believable. The soundtrack itself was pre-recorded, I'll bet to ensure authenticity. Here is an excerpt from an interview with Hulce from People Magazine in December 1984:
Although the movie's sound track was prerecorded with professional musicians, Hulce really is pounding out those notes. "It was my job to have it all seem effortless and believable," Hulce says. "I never knew when the camera was going to be on my fingers." Before going to Prague for filming, Hulce hired a piano teacher. "I spent four weeks, four to five hours a day learning to play. The first two days were scales and exercises. The next day was a concerto." All that hard work had an unexpected payoff. "It helped me understand Mozart," claims Hulce. "How someone could write from morning to dark, then go out and drink and dance and carry on. The energy from my piano playing got me all revved up to go out and trash myself."
The piano(s) used in the film may have had working keyboards, but that doesn't mean they produced sound. A practice keyboard has functioning keys which are weighted to provide the action of a legitimate instrument. Hulce may have been able move his hands and fingers in-synch to the films playback music, without a need to generate a single legitimate note.