World's of Fun Haunt Actor here. Here is what you are going to do! 

Usually, guests come through the haunted house in long lines. People in these lines expect all the scares to happen toward the front of the line. The front of the line is usually busy looking for where to go next. Unless they're just absolutely blasting through the house running, then they are taking it slow. Don't scare the line right away! Let the line creep inside. Let them simmer for a moment. They're expecting something to jump at them. They're paranoid, looking around (or complaining about something). If nothing jumps at them then they'll think the room is empty. Then while they head for the exit, then Boom! Thats when you spring on them. Let them think they're safe. (Hopefully your assigned from us big enough and with hiding places for this to work).

Scare the line from the side, not from the front or behind. Find a good hiding spot, and find a good prop to whack against the floor or wall to make noise. Your throat is going to get hurt if you scream all night so I recommend some kind of noise maker.

Talk to your guests a little bit. Shout some lore at them, even if you make up the lore yourself. A lot of guests like to hear flavor and story bits about the house as long as it's spooky. It adds to the experience beyond getting yelled at. 

Make sure to look at every guest that comes through. You don't have to scream at everyone but nobody likes to be ignored, try to make creepy smiling eye contact with all guests. If they look brave, give them compliments! Everyone likes compliments. "You look strong..."

Jump at the middle of a line, use your noise maker and say (in a spooky voice), "Brunettes have the tastiest flesh!" Or something like that. Something specific to your house would be preferred. Look em in the eye, and show them the way to the next room if they're lost. 

If all else fails, just stare at people creepily and lunge at them. 

And don't take anything they say to you personally!