The Tingler

The 1959 horror film The Tingler involves Vincent Price discovering a parasite that attaches to a person's spine. When they are afraid, the parasite grows stronger, and when they scream, it grows weaker. So it is to the tingler's advantage to scare the victim, and if you can't scream for some reason, you're a goner. But there's more than a monster in this movie- there's murder, too. 

Producer and director William Castle often used gimmicks for his films. For The Tingler, a method he called Percepto! was used. A buzzer device was installed in some theater seats that relayed a "tingle" at certain points int he movie. The audience was encouraged to scream if they felt it, because that would relieve the effects of the parasite. They also staged medical emergencies among some audiences. The Tingler wasn't all that well-received at the time, but has become a cult classic because of its camp qualities and Vincent Price's performance. (Thanks, WTM!)