One of my best memories growing up is a bit of an evil one. One year I got this great idea to do something for Halloween. I decided to dress up as a dummy.
I stuffed a few sets of old clothing with news paper, put a few spare Halloween masks on them, and set them in chairs on the front porch. Then I got some of my baggier clothing, put it on, and stuffed it with news paper as well, so you couldn't tell as easily that I was real. Then I put on a black fabric mask that came with a costume for something like death or a wraith, so people couldn't see my eyes, and put a full headed Nosferatu mask, that was scary just by itself. The mask was latex with gaping maw with fake blood dripping from it. It was VERY scary to a lot of people when they saw it. Then I put a bowl full of candy on another chair, with a sign saying to take some. I snicker even now just thinking about it.
At first I growled, or stood up and screamed scarily, but I found this actually was FAR too scary. One father was soo scared that he ended up running out into the yard, then refused to come back as his son said "It's okay daddy, it's just pretend." The mother giggled as the father yelled that he was never doing this again, and stormed home. Needless to say I realized it was a bit too much, and toned it down a LOT. With younger ones though, it was hard not to scare them too much. Even just seeing me in the mask scared them, without me jumping or anything.
When I did get to pull the full act though, it was great fun. I'd try to hold my breath, and hold as still as I possibly could. Then if I really wanted to mess with them, I'd move my head toward them. They would think their eyes were just messing with them. Then I'd do something to really scare them. I'd growl or roar, or stand up. Even just telling them to just take one candy scared them half to death.
It was soo much fun that I did it quite a few years after.
The real trick was that I had to keep reminding myself to tone it down. People scare far to easily. I could have easily just sat there and scared more than my fair share, but what's the fun in that?