The Chorus is a group of actors that together speak, sing, and dance in one body. The Chorus is part ritual part thematic device that play a much larger role in Greek Tragedy than in the other genres. One of the primary functions of the chorus is to provide atmosphere and, in some ways, underscore the tragic action. When the hero is treading upon major conflict or leading us into the rising action of the plot, the chorus, in a way, are heralders of inevitable disaster and instill a sense of fear or suspense in the audience. In some ways the Chorus can represent the audience's ideal response to the play. Nietzsche feels that the Chorus, and it's chants and songs, helped the audience better connect with the character, revealing the essence of the tragedy.
In early tragedies the Chorus was dominant. They could be used to fill time when the one main actor went off-stage to change roles or for scene changes. The Chorus could sometimes have as much as half the lines in a single show. In Aeschylus' plays, the Chorus could even serve as the protagonist or antagonist.
There is a disagreement among historians as to the actual size of the standard Chorus. Some say it started at 50, then changed to 12 with Aeschylus, then to 15 with Sophocles and Euripides. Those who say the Chorus was always as small as 12 or 15, believe that the 50 Chorus members were divided amongst the four plays with which the author was competing. This helped reduce the cost of a production. Sometimes the Chorus could be as small as three people, or there could be a second Chorus.
The Chorus usually entered with a stately march, sometimes singing. Although, they could enter from multiple directions. Onstage formation would consist of three to five member lines. While the Chorus sung and danced in unison, there could be two groups that took turns. They would exchange dialogue with the characters, sometimes as individuals. When it came to acting, the members of the Chorus would respond appropriately to the situation.
Old Comedy Chorus had 24 members and had approximately two semi-choruses. One of each gender. Chorus in comedies could have mixed genders and were more active. In dramatic form, The Chorus did not enter until after the prologue and stayed onstage for the entirety of the show.
Training of a Chorus was probably spread over the 11 months the playwright had with the Chorus before the performance. At first, it was the author who choreographed the Chorus, but as time went on this job was taken over by professionals. Some say this training was similar to that of athletes.