I'm especially intrigued by the PR stunts and gimmicks used to promote horror movies. The king of these stunts was William Castle. He was responsible for gimmicks like fake skeletons flying over move goers heads, and buzzing the theater seats during the film.
One of the coolest sounding gimmicks ever was for the film The Incredibly Strange Creatures Who Stopped Living and Became Mixed-up Zombies. It was shown in glorious "Hallucinogenic Hypnovision." Now prove me wrong, but it sounds so much more fun to see a movie in Hallucinogenic Hypnovision than George Lucas's THX cinema audio system.
Unfortunately I wasn't born in time to experience these wonders.
The last original horror movie gimmick I can think of came from the obscure low-budget 1978 monster movie Slithis (alternate title: Spawn of the Slithis).
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Here's a UPI photo dated 5/5/78 that shows Slithis at the University of Nebraska. The students in this pic don't seem very impressed.
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Besides the personal appearances, Slithis's big gimmick was an advertised "Survival Kit" handed out ticket buyers.
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If I've whetted your appetite for all things Slithis, check this guys site for an extremely long review of the film. Also here's the Slithis TV spot... Slithis... Slithis ... Slithis...