Thespis of Icaria (6th century BC) is claimed to be the first person ever to appear on stage as an actor in a play. In 534 BC, mostly likely on what would have been November 23rd, Thespis took the stage at the Theatre Dionysus during a choral song and dance, and became the first man to take on the role of a character in a story. Prior to this performance, stories were told in third person narrative only, and no one had ever assumed the resemblance of another person for the purpose of storytelling. By becoming the first actor, Thespis not only created a new art form in acting, but had a substantial hand in changing the way stories were told and inventing theatre as we know it today.

According to Aristotle, writing two hundred years later, Thespis was a singer of dithyrambs (songs about stories from mythology with choric refrains). Thespis supposedly innovated a new style in which one singer or actor performed the words of individual characters in the stories, distinguishing between the characters with the aid of different masks.

It is sometimes implied that Thespis invented acting in the Western world, and that prior to his performances, no one had ever assumed the resemblance of another person for the purpose of storytelling. In fact, it is probable that acting had been in existence for thousands of years, as is indicated by cave paintings such as 'The Sorcerer'. Thespis is, however, the first known actor in written plays, as opposed to improvised or orally transmitted plays. He may thus have had a substantial role in changing the way stories were told and inventing theatre as we know it today. In reverence to Thespis, actors throughout western history have been referred to as thespians.

In theatrical myth (or superstition), Thespis is said to exist now as a mischievous spirit, and when things go wrong in performances it is often blamed on his ghostly intervention. Like many superstitions, this belief ranges in different cases from being considered a humorous legend to being taken very seriously, with various charms and rituals being employed to either invite his approval or defend against him.

In theatre and acting craft, there is a minor school of thought known as "anti-thespian," which posits that it is inappropriate or artistically flawed to assume a character other than yourself. It is not a moral objection, but more a suggestion that actors should not make any effort to disguise or distract from who they are (in theory, because stories themselves are what matter, not the distraction of creating false depictions of people.)

The statement of Horace (Ars Poetica, 276 ) that Thespis went round Attica with a cart, on which his plays were acted is due to a confusion between the origin of tragedy and comedy, and a reminiscence of the scurrilous jests which it was custommary to utter from a waggon (skommata ex amaxes) at certain religious festivals.