dramatis personae (drama) characters in a play (used in old literature, and especially in Shakespeare, before a list of characters)
et tu, Brute? (drama) Even you, Brutus? (used toward any individual who has betrayed another individual in a dramatic and unexpected way).
deus ex machina (drama) god out of the machine (used to criticize poorly written drama where something in the story seems to be suddenly invented just to make the story end happily, like, “… and she woke up to find that it was all only a dream!”
ars gratia artis (drama) art for the sake of art (used to argue that art justifies itself and does not need a moral or logical reason to exist).