Simplify the following text for a Grade 10 native English-speaking person.
Original: The first recorded case of an actor performing took place in 534 BC ( though the changes in calendar over the years make it hard to determine exactly ) when the Greek performer Thespis stepped on to the stage at the Theatre Dionysus and became the first known person to speak words as a character in a play or story .
Simplified: The changes in calendar over the years make it hard to say exactly when the Greek actor Thespis became the first person to speak words as a character in a play at the Theatre Dionysus .