Jim gave you some very good advice. I will add a few things and reiterate a few of his.
Rule no. 1: Keep yourself safe
Rule no. 2: Keep your guest safe
Rule no. 3: Never, ever jump out and say "boo"
Rule no. 4: Don't touch the guests (see discussion of that in pro haunt forum).
Beyond that, as Jim implied, you will have to find a style which works for you. Some of that will be driven by your employer, some by the theme of the haunt, and some by your own personality and skill set. It takes some experimentation. High energy and startle scares can work well. So can very slow movements and very quiet voices. Switching between the two with no notice and get great reactions. Experiment.
Observe. Watch how guests react not only to you, but to other actors. Keep a journal, notebook, or file on computer with those observations. Learn what scares people. Just as importantly, learn what doesn't.
As important as noting what scares people, learn to read people. A solid, experienced haunt actor can almost always predict how a given person will react to a scare and often what kind of scare will work best for them. Learn to read their body language, to listen to what they are saying, to see (if possible) how they reacted to the previous scare.
Learn to attack groups in different ways. Don't always attack the front. Some scares should be directed at the middle or even the back of the group (where the most scared tend to hide).
Lastly, once you get the initial scare, back off, let them feel safe, then go after them again. The second scare can often be better than the first.
Screams are the most addictive substance on earth. Oh, and rule 5: Have fun!
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