How did improvisation work in commedia dell'arte performances?

In commedia dell'arte, improvisation was not entirely spontaneous; rather, it functioned within a structured framework of stock characters, established scenarios, and specialized acting techniques.12

Professional troupes relied on several key elements to facilitate their performances:

• Scenarios and Frameworks: Companies utilized a stock of scenarios—outlines of plot points—rather than full scripts. These provided a firm structure, often borrowing from the classical literary tradition of commedia erudita, while allowing actors the freedom to improvise dialogue and action within those bounds.13
• Lazzi: These were essential improvised comic interludes, consisting of verbal asides, topical jokes, or physical acrobatics. Actors developed a repertoire of lazzi—such as miming or pratfalls—that they could insert into performances to engage the audience or heighten the comedy.41
• Stock Characters: Each actor typically specialized in a single role, such as the witty servant Arlecchino or the pedantic Dottore. By mastering these types, actors developed "commonplace books" of soliloquies and witty exchanges, which they adapted to fit the specific flow of each performance.1

Over time, repeated performances led to the development of standard speeches and stage business, blending creative improvisation with a reliable, recognizable theatrical form.2