"So Thespis is the Roman god of theatre?"
"That's exactly right. Except he's not Roman and he's not a god."

Aaron Sorkin, Sports Night

Thespis is a tricky little brat, if you want the truth.

The truth to Thespis' origin in complicated and contradictory, so I'm going to go with the myth - Thespis is widely regarded to be the father of all theater, and is credited, in 534 BCE, with being the first person to take to a stage and speak dramatically. Hence Thespians, which is pronounced way too much like Lesbians with an effeminate lisp not to be funny. He was Greek, not Roman, and very mortal. Nowadays, he usually spends his time as a ghost of the mischievous variety, wrecking havoc of performances of any kind - screw-ups that happen onstage in rapid succession with no discernible cause are said to be the work of Thespis.

Actors are a superstitious bunch, and they take their ghosts seriously. If beset by an itinerant ghost, knowledgeable sources say that it's best to just soldier on and to not panic - you might end up with a fiasco, but you'll at least come out of the evening with your sanity intact.

It's also important to note that Thespis' influence rarely crosses the line into the truly dangerous or deadly - he'll ruin a performance but will generally stop short from actually hurting anybody. I'm sure the actor types out there have a wholly different spirit in their extensive pantheon who takes care of the actual maiming, but Thespis is just in it for the laughs.

One date taken from Wikipedia's entry here:
The rest's from somewhere in my addled little brain