Well...are you crashing the party, so to speak, or are you a legit invite? I suppose it doesn't much matter, does it? Is the expectation that you dress like a character from the movie, or just come as you are (and happen to be in a kilt)? I suspect that since you will be throwing, an outfit suitable to throwing is in order...I might step on toes suggesting that you go in a character that might have been in the movie, but there you go. I would imagine that is what they expect.