We're continuing the discuss the rules for dressing up to go to a movie. So far, we've said that you should....
#1 - Go with friends.
#2 - Realize that it takes guts
#3 - Measure the likelihood that others will dress up
Rule #4 is that the "bigger" the movie the safer you are to dress up. Fans didn't think twice about dressing up for Harry Potter and the Deathly Hallows Part 2. Why? Because this film was bigger than the Pharaoh in Egypt! Fans had attended movie theaters for seven previous films and seen other fans dressed up. At that point, Harry Potter was a very well-established and hotly anticipated property. In many regards, it was expected that fans show up en masse.
While this might seem like it's the same as rule #3, it's not. The likelihood that fans will dress up for Men in Black III is low but that film is going to be huge no matter what so, although you may be the only fan dressed up, you're still okay to go in costume. Chances are the other fans will applaud your courage and creativity.
When the film has a big buzz around it or is much-anticipated it becomes much more socially acceptable to dress up when you attend. It's a numbers game - a packed theater is much more accommodating to a lone fan who shows up in costume than, say, a show where it's you and three other dudes. So while you may be the only one, it's cool if the theater is packed. In the case of a big movie event, you may even have other fans wishing that they had thought of dressing up as well.