Improvisation
The singer was often expected to improvise variations and ornaments during the third section, to keep it from being a mere repetition of the first. This was especially so for da capo arias written in slower tempos, where the opportunity to improvise, as well as the risk of dullness, were greater. The ability to improvise variations and ornaments was a skill learned by, and expected of, all solo singers. The decline in this ability following the Baroque era is perhaps the reason why the da capo aria ultimately acquired a reputation as a musically dull form. The authentic performance movement, starting in the mid twentieth century, restored improvisation to the performance of da capo arias, although the practice has yet to become universal even among authentic performance artists.
Read more about this topic: Da Capo Aria