To summarize, we're detailing the rules you need to follow should you want to dress up in costume or bring props to a movie. So far, we've said the rules are...
#1 - Go with friends
#2 - Know you're going to get some odd looks
Rule #3 is to anticipate the crowd. By that, we mean measure the likelyhood that others will dress in costume as well. If it's Harry Potter, X-Men, Twilight, Star Wars, Star Trek, or Rocky Horror Picture Show, I think you're very, very safe assuming that a plethora of others will be in costume as well. But these are all established properties. You might be the biggest Smurfs fan out there but I can guarantee you you'll be the only one dress up as Papa Smurf. And that just makes it a little awkward, weird, and creepy. And nobody wants that.
Avoid being the only one to dress in costume. Not that there's anything wrong with going against the grain and letting you're fan-freak flag fly. But perhaps a convention is a more appropriate venue to unleash your really cool ape costume from Rise of the Planet of the Apes. Take my word on this one.