The primacy of the composer's written score has also led, today, to a relatively minor role played by improvisation in classical music, in sharp contrast to the practice of musicians who lived during the baroque, classical and romantic era. Improvisation in classical music performance was common during both the Baroque and early romantic eras, yet lessened strongly during the second half of the 19th and in the 20th centuries. During the classical era, Mozart and Beethoven often improvised the cadenzas to their piano concertos (and thereby encouraged others to do so), but they also provided written cadenzas for use by other soloists. In opera, the practice of singing strictly by the score, i.e. come scritto, was famously propagated by soprano Maria Callas, who called this practice 'straitjacketing' and implied that it allows the intention of the composer to be understood better, especially during studying the music for the first time.
What part did Mozart and Beethoven often improvise?
the cadenzas to their piano concertos