Not a horror/slasher movie, but I remember the first time I watched The Godfather. Went to the theater with my fiance at the time. Had read about the horse head under the sheets scene. So when it was about to show that, I asked her if she wanted anything from the concession and got up and left the theater. Hardly could contain my laughter as I was ordering popcorn and a drink. When I got back and sat down she was like Sawnee's wife...white as a ghost. When I started laughing she knew what I had done. Never been hit so hard in my life.
Friend from Illinois told me a story about going to see psycho 3 when it was released in the theater. He went to the movie with one girl (they were friends and used to go to movies together quite a bit) on a Thursday night. He was supposed to go see it with his buddies that weekend, but forgot and took a date to see it. He was sitting 4 rows in front of his guys and didn't know it...
Anyway, here's the rest cut and pasted-
"...the girl I was seeing was really into scary movies, so I had to go. Again. The first showing was boring, not like the first Psycho which was at least interesting and well filmed. To sit through it again, I needed to make it more interesting.
There was a scene with a clock at the end of a hallway, telegraphed so much that when the character bumped into it, I nudged the girl the first night and she jumped, but laughed about it pretty good.
The next viewing I waited until that moment, made sure there wasn't anyone I might scare TOO MUCH, and yelled a little. My girlfriend jumped out of her seat, my buddies jumped out of their seats, and about 8 rows up a woman screamed and popcorn went flying everywhere.
For the rest of the movie my buddies threw popcorn at my head, and the husband of the woman up front kept laughing and giggling through the entire film, with his wife threatening him every few minutes.
Almost the most fun I ever had at a movie."
I asked if his date with the girl was his last with her and he said he couldn't recall. I wonder why that is...