The autograph manuscript shows the finished and orchestrated Introit in Mozart's hand, as well as detailed drafts of the Kyrie and the sequence Dies Irae as far as the first eight bars of the "Lacrymosa" movement, and the Offertory. It cannot be shown to what extent Süssmayr may have depended on now lost "scraps of paper" for the remainder; he later claimed the Sanctus and Agnus Dei as his own. Walsegg probably intended to pass the Requiem off as his own composition, as he is known to have done with other works. This plan was frustrated by a public benefit performance for Mozart's widow Constanze. Constanze was responsible for a number of stories surrounding the composition of the work, including the claims that Mozart received the commission from a mysterious messenger who did not reveal the commissioner's identity, and that Mozart came to believe that he was writing the requiem for his own funeral.
The Requiem is scored for soprano, contralto, tenor, and bass soloists and an SATB mixed choir.