An actor — someone who acts in plays or films. It is a fancy, old-fashioned word, and that is the whole point of it. Nobody in normal conversation says "my brother is a thespian" the way they would say "my brother is an actor." It shows up in two places. First, in formal or literary writing — a theatre review, an award speech, a school drama club's name — where it sounds grand and respectful, and carries a hint of serious stage acting rather than film or TV. Second, and more often these days, as a gentle joke. If a friend makes a big dramatic show of being tired, you might call them a thespian, meaning "look at you, performing." The joke works because the word is so much bigger than the moment. The same word also works as an adjective, meaning "to do with acting or the theatre" — thespian talent, thespian ambitions — though this is rarer and sounds even more bookish. When it is written with a capital T (Thespian), it usually points back to the Greek origin or to a named drama society. Note that it is a neutral-to-warm word, not an insult: even when used jokingly, it teases rather than attacks.