The truth of the matter is that no matter how carefully you plan your haunted house, something is bound to go wrong as soon as the guests appear. It can be as accidental as the guests noticing some problem that you and the rest of the staff managed to completely overlook, or it might be as deliberate as the trashing of the sets that occurred on Halloween at the Bayou Brewing Company's haunted house. Guests can be mirthful or terrified, skeptical or gullible, overprotective or downright dangerous. You have to be aware of the possibilities and how to minimize danger.
First of all, there are two forms of haunted house: controlled and uncontrolled. An uncontrolled house is a bunch of set pieces that guests are allowed to freely wander through. This works pretty well for houses geared towards kids under 12 but can be a preparation for disaster with older groups. The aforementioned Bayou Brewing Company house was uncontrolled; though the maze was set up to push the guests in one direction, anyone who wished to loiter could do so unimpeded, and anyone who wanted to put a foot through a piece of panelling could do so in relative privacy with no fear of getting caught.
A controlled house is one where the guests are led through the house, such as the "Aliens" setup I described earlier. This usually requires a leader and a follower; the follower is specifically charged with keeping the stragglers up and with making sure any children or adults who need to leave suddenly for any reason are escorted to the nearest exit. This has the benefit of reducing vandalism but requires a larger manpower commitment.
In a controlled or uncontrolled environment, one thing must be taken into consideration: You are not allowed to touch the guests. Ever. Not to grab them or to scare them; not to tap them on the shoulder; not to stop them from kicking an antique into pieces. The unfortunate truth of the matter is that in our lawsuit-happy culture, even the slightest touch might prove the fodder for a personal injury lawsuit. Don't give yourself that kind of grief.
However, looming, screaming, growling, and general menace are all allowed. And, naturally, if there is a medical or safety issue, you are allowed to intervene as would any bystander. If you are really concerned about vandalism, consider hiring a security guard who can take care of troublemakers in ways that you cannot.
Of course, there is always the possibility of unintentional damage. A frightened person might run against a wall, or over a prop. The only thing to do in such circumstances is to back off and let the person calm down. If the guest gets hysterical, call for a plainclothes staff member to escort the guest out. (A costumed character might frighten the person more.) This is a rare occurence but one that should be considered.
Above all, be flexible, and react to the guests as much as possible. An interactive haunted house is more interesting than a static museum setup.
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