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Far as I'm aware, theatre is UK English and theater, US. Certainly, theatre is most-used in the UK and not just by those in the business.
It can also be the type of theater, too. Or maybe I'm just weird, going to the movie theater, and going to the theatre to see a play.

"You need me. You wouldn't be much of a hero without a villain. And you do love being the hero, don't you. The cheering children, the swooning women, you love it so much, it's made you my most reliable accomplice." -- Lex Luthor to Superman, Question Authority, Justice League Unlimited