This is one of my favorite scenes from Amadeus. It is rare for a film to have two actors be nominated for the same award. Thomas Hulce eventually lost to F. Murray Abraham at the Oscars, but there is no denying that both gave outstanding performances.
It was said that the real Mozart had a very childlike laugh which Hulce does his best imaginative version of. The laugh is so jarring and out of the blue that it catches the Emperor off guard twice.
Mozart is either arrogant or socially unaware because he insults Salieri multiple times. As Mozart plays Salieri's little march and notes "the rest is just the same, isn't it?" you can see how maligned Salieri felt. Then to top it off Mozart declares "that doesn't quite work, now does it?" and then proceeds to take Salieri's tune and add embellishments and improvisation such that Salieri can only look on in jealousy mixed with resentment.
Salieri's jealousy and contempt for Mozart is best summed up with the line "Grazie, Signore..." as he looks up at the crucifix.