Thespis

In about 600 BC, Arion of Mehtymna ( Corinth ) wrote down formal lyrics for the dithyramb. Some time during the next 75 years, Thespis of Attica added an actor who interacted with the chorus. 

This actor was called the protagonist, from which the modern word protagonist is derived, meaning the main character of a drama. Introduce a second speaker and one moves from one art, that of choric chant, to another, theatre. 

Tradition ascribes this innovation to one Thespis and even gives him a date; he is said to have performed Athens about 534 BC. Whether this is true of not, his name has achieved immortality in theatrical jargon - 'actors' and 'Thespians' are synonymous.