Added information by sillygoldfish:
The role of the greek chorus was:
to offer a sense of "rich spectacle"
to provide time for scene changes
to give the main actors a break
to offer background information
to offer information about the main themes animating the action
to show an ideal audience's response
to act as a crowd
to tell of battles (battles, murders etc. were not allowed to be performed in greek theatre)
to be a go-between from the audience to actors
to tell the audience of character's fear and secrets (what the actors couldn't say themselves)