While there's no definitive proof, Thespis is credited with introducing the first actor to Athenian drama in the 6th century BC. Before him, plays were simply choral performances. Thespis is said to have introduced a single actor, who interacted with the chorus and played multiple roles. This innovation is considered a crucial step in the development of Western theatre.
The term "Thespian" even comes from his name, and it is still used today to refer to an actor.