Contrary to what you may think, there was a great deal of play, creativity, and improvisation in the music of the Baroque era.
Think about it: when your score basically just indicates the rhythm and gives an outline of some harmonic changes, the composer is leaving it up to you, the performer, to fill in the blanks. And the composer is counting on you to make it emotionally expressive, exciting, and highly theatrical.
This is a page from Giulio Caccini’s book Le nuove musiche (The New Music), with the notation for his song “Amarilli, mia bella.” Note that the vocal line is extremely simple and straightforward, and that the harmonies are not written in. This is because Caccini expected the performers to do the rest. In his book, he describes various ornaments, how to execute them, and where in the text to use them most effectively.
For instance, here Caccini explains a trillo and a gruppo:
Or, in modern notation:
These ornaments are meant to advance the emotional content of the music, so that the score, when performed, can sound like this:
Or like this:
So, at the same time that music is beginning to conform more to certain structures, it remains incredibly free within those structures.
The ensemble early music ensemble L’Arpeggiata routinely improvises onstage. Here they perform one of Monteverdi’s madrigals, with each of the singers striving to outdo the other with improvised ornaments — sort of like a “cutting contest” in jazz.
The instrumental players also perform improvised solos, not unlike in dixieland jazz, where all the musicians “solo” (i.e. improvise melodic lines) at the same time.