Prior to the plays of Aeschylus (c. 524 –c.455 BCE), ancient Greek plays of the sixth and fifth centuries BCE had a large chorus of up to fifty members who interacted with a single actor. The first actor in ancient Greek plays is believed to have been Thespis, in whose honor actors are referred to as "thespians."
Since the single actor had no one else with whom to interact, the role of the chorus was nearly equal to that of the single actor. The chorus engaged in dialogue with the actor either as a whole, or through the chorus leader, called the coryphaeus.
Aeschylus is credited with adding a second actor to his plays, and he also reduced the size of the chorus from fifty to twelve.
The chorus plays a significant role in Aeschylus's tragedy The Suppliants, (also known as The Suppliant Maidens, or The Suppliant Women), and in Prometheus Bound, which until the 1800s was believed to have been written by Aeschylus, but the authorship has since been in doubt.
In The Suppliants, the chorus is composed of the suppliant women themselves, the Danaids, who are fleeing forced marriages to their Egyptian cousins.
In Prometheus Bound, the chorus is composed of the daughters of Oceanus, the god of the sea. Because Prometheus is immobilized throughout the play, the play consists almost entirely of spoken dialogue, much of which is between Prometheus and the chorus.
The chorus takes an active role in each play. In The Suppliants, the chorus is the collective protagonist of the play. It is their fate that's determined during the course of the play, and they're ultimately afforded protection from the Egyptians by Pelasgus, the King of Argos.
The chorus in Prometheus Bound has less of a stake in the action of the play, but the chorus nonetheless takes an active role in sympathizing with Prometheus's plight, and in offering him advice, solace, companionship, and protection—at least until the end of the play, when Zeus strikes Prometheus with a thunderbolt, casting Prometheus and the chorus into the abyss.
It wasn't until Sophocles (c. 496–406 BCE) added a third actor to his plays that the role of the chorus was reduced to providing commentary on the action of the play, rather than taking an active role in the play itself.