I've seen it twice. Last time (just a few weeks ago), I dressed as Riff-Raff and hubby went as Frank N Furter. The audience was expected to behave but were still allowed to have lots of fun. Lots of cat-calling from certain sections of the audience (you can shout '****' whenever they say Janet's name, and 'a**hole' whenever they say Brad's), lots of singing along and clapping, and everyone standing up to do the time warp in the aisles at the end.
There was a general atmosphere of fun in the whole building, and lots of people were wearing ridiculous outfits, so it's unlikely you'll be the only one.
Just go and really enjoy yourself. It's great fun.
Oh, and you might be better to take a small torch than have to use a lighter - some theatres are quite strict on the fire thing.
Do you know who's going to be in it?