This happened several years ago when I managed a movie theatre. Most people are unaware of this, but the screen in a theatre is perforated, otherwise the heat from the projector is hot enough to set the screen on fire, but the holes prevent this from happening. So you can be behind the screen and see all the people watching the movie.
One night my girlfriend and I went behind the screen and she jacked my dick while we watched the audience. I don't think I ever came so hard in my life. It was a real kick knowing we could see all those people, but they could not see us. Several times we would get naked and masterbate either ourselves or each other. Once we got two other couples to join us, all six of us were naked and sitting in a chair facing the audience and masterbating, what a rush. Next time you are at the movies think what might be going on behind the screen!!