How did early Baroque sinfonias function in opera and theatrical productions?

In the early Baroque period, the term sinfonia was used to describe instrumental pieces that functioned as independent works or as essential components within theatrical productions, including operas and oratorios.1

In the context of opera and theater, these sinfonias served several specific roles:

• Introductions: They were frequently used as orchestral overtures to open an opera or cantata, setting the stage for the performance.21
• Interludes: They functioned as instrumental passages between scenes or acts, providing musical transitions within the drama.1
• Descriptive Accompaniment: Composers used them to underscore specific staged events. For example, Claudio Monteverdi utilized a sinfonia da guerra ("sinfonia of war") to accompany a battle scene in his opera Il ritorno d’Ulisse in patria.1
• Structural Variety: Early examples often featured a sectional form characterized by contrasting meters and tempos. By the late 17th century, Alessandro Scarlatti helped formalize the sinfonia avanti l’opera into a standard three-part fast-slow-fast structure, which eventually evolved into the autonomous orchestral symphony.12