No, composer Salieri didn’t really try to swindle Mozart out of the Requiem.
As the screenwriter for the movie Amadeus Peter Shaffer puts it, “It’s a fantasia based on fact. It is not a screen biography of Mozart and was never intended to be.”
Antonio Salieri was a contemporary of Mozart and admired his work very much. However well they knew each other, Mozart did not become dependent on him, and Salieri wasn’t at Mozart’s deathbed.
The Requiem was actually written for Count von Walsegg zu Stuppach, a rich amateur musician. The count liked to throw lavish parties and play unsigned scores to see if his guests could guess the composer who had written it, oftentimes passing the works off as his own.
An employee of the count’s, Anton Herzog, recounted these scenes as such:
The secretly organized scores he generally copied out in his own hand, and presented them for the parts to be copied out. We never saw an original score. The quartets were then played, and we had to guess who the composer was.
Usually we suggested it was the Count himself, because from time to time he actually composed some small things; he smiled and was pleased that we (as he thought) had been mystified; but we were amused that he took us for such simpletons.
We were all young and thought this an innocent pleasure which we gave our lord. And in such fashion the mystifications continued among us for some years.
The count also requested that the piece be such that it could be played at his recently deceased wife’s memorial as well—killing two birds with one stone, as it were. When Mozart passed away, the Requiem was left partially unfinished with only two movements left, so Constanze, Mozart’s wife, delivered the piece and was paid by Count von Walsegg.
Furthermore, she asked Mozart’s student Franz Siissmayer to finish the Requiem so she could publish it and get some royalties from her dead husband’s piece. Siissmayer did his best to finish it in Mozart’s style.
However, the film’s portrayal of Mozart’s delirium was somewhat accurate. He was dying during the writing of the Requiem, and he did suffer from high fever and hallucinations.
At one point, he offhandedly commented that he felt he was writing his own Requiem. And in a way he was:
The unfinished Requiem was performed, unbeknownst to the Count, at Mozart’s memorial.