Characteristics of the buildings The plays had a chorus from 12 to 15 people, who performed the plays in verse accompanied by music, beginning in the morning and lasting until the evening. The performance space was a simple circular space, the orchestra, where the chorus danced and sang. The orchestra, which had an average diameter of 78 feet, was situated on a flattened terrace at the foot of a hill, the slope of which produced a natural theatron, literally "watching place". Later, the term "theater" came to be applied to the whole area of theatron, orchestra, and skené. The choregos was the head chorus member who could enter the story as a character able to interact with the characters of a play. The theaters 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. Mathematics played a large role in the construction of these theaters, as their designers had to be able to create acoustics in them such that the actors' voices could be heard throughout the theater, including the very top row of seats. The Greeks' understanding of acoustics compares very favorably with the current state of the art, as even with the invention of microphones, there are very few modern large theaters that have truly good acoustics. The first seats in Greek theaters (other than just sitting on the ground) were wooden, but around 499 BC the practice of inlaying stone blocks into the side of the hill to create permanent, stable seating became more common. They were called the "prohedria" and reserved for priests and a few most respected citizens.
In 465 BC, the playwrights began using a backdrop or scenic wall, which hung or stood behind the orchestra, which also served as an area where actors could change their costumes. It was known as the skênê (from which the word "scene" derives). The death of a character was always heard behind the skênê, for it was considered inappropriate to show a killing in view of the audience. In 425 BC a stone scene wall, called a paraskenia, became a common supplement to skênê in the theaters. A paraskenia was a long wall with projecting sides, which may have had doorways for entrances and exits. Just behind the paraskenia was the proskenion. The proskenion ("in front of the scene") was columned, and was similar to the modern day proscenium.
Greek theaters also had tall arched entrances called parodoi or eisodoi, through which actors and chorus members entered and exited the orchestra. By the end of the 5th century BC, around the time of the Peloponnesian War, the skênê, the back wall, was two stories high. The upper story was called the episkenion. Some theaters also had a raised speaking place on the orchestra called the logeion.(wikipedia)