Back in my youth, I understood that if you paid for one movie showing you could stay and watch the same movie as many times as you want.
Is this still the case?
I dont know where you are, so why don’t you try it? Here in California they don’t let you do that, they have someone come in & clear the room, unless it looks like very few people are watching said flick.
We used to do this all the time, especially if we missed the first few minutes of the feature. This was in the days after they showed a cartoon, but before they had a dozen previews before the movie. Also, the theatre was a single screen venue.
When we did get twin theatres, they were built so that the consession stand split the lobby in two, so people couldn’t see the other movie after they’d seen the first. They would still let us sit and start to watch the next screening, if the movie wasn’t expected to pack the theatre.
With the megaplexes, it’s too easy to jump between screens. Even with tickets as overpriced as they are, theatres would start to lose money if they didn’t clear the room after each screening.
It can be done, but you have to be subtle. (Not that I’ve done it.)
[ul][]Don’t go to the consession stand too much, if at all.[]Don’t be obnoxious (this is what usually gets teens caught)[]Wear generic clothes[]Have a generic haircut (Billy Idol spiked hairdo’s are out)[]Hang out in the restroom between features, so you don’t sit in the new theatre too long before it starts[]Don’t repeat this list to anyone 
[/ul]
I don’t know how old you are but the problem with doing that today is that the next show may be full and the people who paid for seats during that show should be able to get one without you (who didn’t pay for a seat during that show) taking one up.
With today’s smaller auditoriums sell outs happen more often so the theatre manager will have to boot you but if it’s a slow day they probably wouldn’t mind as long as you didn’t change movies.