What is the role of the Choragos, or chorus leader? Why are there times when only the Choragos speaks, and not the entire chorus?

Answer 1

http://en.wikipedia.org/wiki/Greek_theatre
Scroll down to Characteristics of the building and read about the choragos toward the end of the first paragraph of that section.

Also this: http://www.answers.com/choragos

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Answer 2

define choragos