It is a fair assumption that Mozart expected to be in contact with his singers during the process of aria composition. In a famous passage concerning a piece he was composing for the singer Anton Raaff, he wrote:

In his letters Mozart made detailed appraisals of several of his singers, and it is clear that he studied carefully the strengths, weaknesses and vocal characteristics of anyone for whom he wrote. In Raaff 's case, he noted with approval several positive features, such as his impressive diction and good breath control.

In order to ensure that the singer was happy with what he had written, Mozart’s usual practice was to run through an aria with its performer before committing himself to a conclusion, leaving open the possibility of lengthening or abbreviating the piece. In several arias in Così, there is in fact a break in the ink colour of the particella shortly before the end of the singer's material. Conceptually, such a draft would be more open-ended than, for example, an aria particella which merely lacked its final orchestral ritornello. After glancing through the draft or trying it out with the composer accompanying at the keyboard, a troublesome singer might reject the proposed aria entirely, in which case Mozart would be left with several sheets of paper, lightly scored but unusable.