This recently came up in the office and included a heated debate between a person who fancies themself as a thespian and a former movie theater employee.
Thank goodness for Google, or this argument might have really gotten out of hand. The following is a generalization. I’ll even provide a few examples when the generalization doesn’t hold.
Theater is a location. As in the Stadium Movie Theater. The Drive-In Theater. Unfortunately, my next example, Akron Civic Theatre, throws the whole thing off.
Theatre is the act of theater. As in a play, a musical, a live performance.
The more I delved into this topic (obviously after the argument was settled), the more I realized that the difference seems to be more preference than anything else.
So whether you write theatre or theater, just be consistent.