(A summary from Greek Theatre.com)
Ancient Greek Theatre
The Greek theatre history began with festivals honoring their gods. A god, Dionysus, was honored with a festival called by "City Dionysia". In Athens, during this festival, men used to perform songs to welcome Dionysus. Plays were only presented at City Dionysia festival.
Athens was the main center for these theatrical traditions. Athenians spread these festivals to its numerous allies in order to promote a common identity.
At the early Greek festivals, the actors, directors, and dramatists were all the same person. After some time, only three actors were allowed to perform in each play. Later few non-speaking roles were allowed to perform on-stage. Due to limited number of actors allowed on-stage, the chorus evolved into a very active part of Greek theatre. Music was often played during the chorus' delivery of its lines.
Tragedy, comedy, and satyr plays were the theatrical forms.
Tragedy and comedy were viewed as completely separate genres. Satyr plays dealt with the mythological subject in comic manner. Aristotle's Poetics sets out a thesis about the perfect structure for tragedy.
Thespis is considered to be the first Greek "actor" and originator of tragedy (which means "goat song", perhaps referring to goats sacrificed to Dionysus before performances, or to goat-skins worn by the performers.) However, his importance is disputed, and Thespis is sometimes listed as late as sixteenth in the chronological order of Greek tragedians.
Aristotle's Poetics contain the earliest known theory about the origins of Greek theatre. He says that tragedy evolved from dithyrambs, songs sung in praise of Dionysus at the Dionysia each year. The dithyrambs may have begun as frenzied improvisations but in the 600s BC, the poet Arion is credited with developing the dithyramb into a formalized narrative sung by a chorus.
Three well-known Greek tragedy playwrights of the fifth century are Sophocles, Euripides and Aeschylus.
Comedy was also an important part of ancient Greek theatre. Comedy plays were derived from imitation; there are no traces of its origin. Aristophanes wrote most of the comedy plays. Out of these 11 plays survived - Lysistrata, a humorous tale about a strong woman who leads a female coalition to end war in Greece.
Theatre buildings were called a theatron. The theaters were large, open-air structures constructed on the slopes of hills. They consisted of three main elements: the orchestra, the skene, and the audience.
Orchestra: A large circular or rectangular area at the center part of the theatre, where the play, dance, religious rites, acting used to take place.
Skene: A large rectangular building situated behind the orchestra, used as a backstage. Actors could change their costumes and masks. Earlier the skene was a tent or hut, later it became a permanent stone structure. These structures were sometimes painted to serve as backdrops.
Theatron: Rising from the circle of the orchestra was the audience. The theatres were originally built on a very large scale to accommodate the large number of people on stage, as well as the large number of people in the audience, up to fourteen thousand.
The cast of a Greek play in the Dionysia was comprised of amateurs, not professionals (all male).
Ancient Greek actors had to gesture grandly so that the entire audience could see and hear the story. However most Greek theatres were cleverly constructed to transmit even the smallest sound to any seat.
Costumes and Masks
The actors were so far away from the audience that without the aid of exaggerated costumes and masks.
The masks were made of linen or cork, so none have survived. Tragic masks carried mournful or pained expressions, while comic masks were smiling or leering.
The shape of the mask amplified the actor's voice, making his words easier for the audience to hear.
Here is a wonderful video introduction to Greek Theatre, both Tragedy and Comedy: