I went to Book of Mormon's final dress rehearsal, and basically Casey Nicholaw got up and made a speech prior to the show beginning that this was a dress rehearsal so there could be stops and starts here and there, so to expect that. I will say though, they never did stop, and there was little that actually would have prompted them to do so, the only thing I remember was a minor set issue, which worked itself out, and as I mentioned they never stopped.
You won't receive a Playbill like you would attending something in previews or that's opened, and you're usually in the company of family/friends of the cast and the creative team. The Book of Mormon final dress rehearsal allowed Facebook fans [such as myself] to attend after winning a contest, which is how I got to go. It was a cool experience, and it's also pretty neat to say I was one of the first audiences to see it.
"Anybody that goes to the theater, I think we’re all misfits, so we ended up on stage or in the audience.” --- Patti LuPone.