When I saw Slithis, they had Slithis (or at least a man in the rubber Slithis costume) in the lobby. Not sure if it made the movie any better.
Also, when I saw Beneath the Planet of the Apes, they passed out souvenir newspapers, one side was the issue for ape city, the other was for mutant city.
I haven't seen it for years, I suppose I lost it. it was a pretty cool item.
Supposedly, there were some cast members in the theater when I saw Supercop, not really sure if it was some of the cast or not, the movie is not very memorable for me in any regard.
I've been to Rocky Horror with audience participation a few times, that would be about the best.
A showing of The Omen I attended had some one make an evil moaning sound in the back row at a crucial point in the movie, scared the crap out of the audience.