I worked in a haunted house when I was in high school. One night I was the caretaker for a girl in a casket, to make sure no one "messed" with her. We knew that some people would lean over the casket and that was OK, but we needed something for extreme occasions.
An air horn setup in the coffin did the trick nicely. Had the compressor in the next room, girl had the control switch (and earplugs, as the horns were at her shoulder, pointed up). It was a treat to see bravado change instantly to a near heart attack!