I have been to a few horror movies where some people laugh. I don't know if its their reaction to being scared or if they are just being immature... or they find the movie funny. Anyway, hard to get into a horror flick with people laughing at the scary parts.
Once there were a bunch of teenagers laughing and crap when I was on a date with a girl who is now my wife of 12 years. We were trying to watch he re-make of the Texas Chainsaw massacre. They were sitting right in front of us and I asked them to quiet down. They stifled some laughs for a few minutes but then it got worse. And then a few minutes later when spooky music started playing I leaned in between the group and whispered in my best Clint Eastwood voice "I just got out of prison today and you kids are about to be starring in your own horror flick if you don't shut your f#@Kin' pie holes"... From then on as quiet as mice but there was a bit of a ammonia scent we had to deal with. lol... just kidding.... It was just a few years ago watching Frozen and the kids were probably just out of kindergarten but they did quiet down and one definitely peed themselves... but I think that was the parent.
Honestly, I have never said anything to anyone at the theater other than once I asked a guy if he could keep it down a bit and he was cool the rest of the movie. But that was because I was on a date and the girl I was with was bothered by it. If I was by myself or with friends I probably wouldn't have said a thing. I simply don't like having to deal with it. I have moved a few times. I'm just not someone who doesn't mind conflict and avoid it when I can...