I know that Amadeus has nothing to do with the actual Mozart, but one thing I did think the movie version got right -- maybe in spite of itself -- is the fact that Mozart was, perhaps above all, a composer with a real feel for theatre. I'm thinking of the scene where he gets excited in talking about the long second-act finale of The Marriage of Figaro and explaining how he managed to carry off a 20-minute comedy sequence in music. Other composers knew how do do drama or character in music, but Mozart may have been the first opera composer to really understand how music works with stage action and makes it better and funnier. And one reason why his best operas work so well in performance, and are so hard for directors to kill, is that the timing and pacing of the scenes is inherent in the music and can't be wrecked even by bad staging.
I was thinking about this when I came upon this clip from the 2006 Covent Garden production of Figaro (with Canadian Gerald Finley as the Count), with one of the first big "plot" numbers in the opera, the trio for the Count, Susanna and Basilio. The number doesn't start until 3:55 in this video -- what comes before is recitative with harpsichord, which in Italian comic operas is the equivalent of spoken dialogue in German and French comic opera. (Figaro has quite a lot of recitative because it has so much plot that has to be explained quickly before it can move on to a number.) Anyway, this four-minute number is a milestone in the history of theatre because it was one of the first musical numbers where the stage action didn't stop: usually the plot exposition ends with the recitative and the number reflects on what happened, but instead the number just keeps going with the plot and builds to the big surefire laugh of the opera (and the original play).
But what's really impressive is the way Mozart manages to get so much into only four minutes, and use music to make the scene even funnier than it was in the play. First off he has different themes and emotional states for each character (and the Count and Basilio reprise their themes at different points, which rounds the whole number off and gives it a formal shape). Then he throws in musical cues that are so specific that they drive the stage action: the bit where Susanna pretends to faint, and the Count and Basilio use it as an excuse to feel her up (I'm sorry: listen to her heartbeat) is the same in almost every staging, even "revisionist" stagings, because the music is so sleazy at that point that the director really has only one way to play it. The joke of having Basilio repeat "Ah, what I said about the page was only my suspicion," the same music and words with two different meanings at two different parts of the number, is a joke that Mozart added (it was not in the original libretto). The Count also briefly starts singing Basilio's theme when he's telling his story of the last time he was sneaking around, which, I recall a critic pointing out, establishes in a few seconds that he uses the same underhanded methods as his shifty henchman. And for the big reveal, Mozart may have been the first composer to figure out how to time for laughs: the music almost fades away to nothing, just enough to keep the scene going but timed just right to get the laugh, milk the laugh, and segue into the end of the number.
A lot of good composers didn't really have this feel for theatre. Wagner believed that his operas were the greatest achievements in theatre ever, and he did try to time his music to stage action, but a lot of his music is not all that well timed, often too long or short or fast or slow for the action he's calling for.