I once had a player who was 20 points north of genius, and he knew it. I wouldn't say he was arrogant but when it came to gaming this guy was utterly ruthless. Every time our group picked up any kind of new game he had it mastered inside of four days regardless of what it was. Eventually the only things we'd agree to play with him were dice games (RPGs and 40k mostly).
So one day we were playing a homebrewed fantasy RP and I was the GM. He had been getting cocky lately because he could pick apart every little thing I'd throw at the party that wasn't combat. It was getting tough to write material that was.... fun for ALL of us and I'd had enough. I sat down and gave it some thought and eventually came up with a solution: I was going to use 'Silent Hill' like a marinade and soak our next session in it for about a week and see what happened.
So I send them into an abandoned outpost off in the trees to find out why the palace lost communications. When they get there the trademark fog settles in and reality turns inside out. I had very specific puzzles planned out but the solutions were almost entirely ‘intuitive’ as opposed to logical. There was very little combat and I did my best to screw with their heads. The best part was only ONE of them had seen the movie (Silent Hill) or played the games and he kept his mouth shut and just laughed as his party-mates were all reduced to gibbering idiots.
Mr Genius was the most satisfying reaction.
The main puzzle of the game was to try to save a five-year-old girl. Removing her from the mist was the solution… but she was also the cause
of the mist. What did the brain of the group figure out?
“She’s the cause of this demon mist. I run her through.”
I signaled my special effects guy in the back of the room and he hit a soundboard for the air raid siren and then dimmed the lights in the living room to nothing. I rearranged the table in the dark (more or less at random) and lit two candles that I’d placed atop what were to be giant rusty pyres. I explained what everybody was looking at (a world of rusty metal, bloody canvas body bags, fire, and unending darkness). There was a round of collective ‘gulps’. Mr Genius went pale, got up, and went outside to smoke a cigarette. This guy doesn’t smoke.
There is nothing more satisfying as a GM than the first time you provoke an intense, and genuine emotional reaction from a player, especially fear.