Why scare the shit out of someone?
Before I begin, I'd like to address the question "Why would one want to scare the shit out of someone else?" There are various reasons, of course, but the best one is this: fear and excitement go hand in hand. It is for this reason that we shell out ridiculous amounts of money to go to amusement parks, which would be better referred to as "embrace your fragile mortality parks." For the most part, there is little risk involved in riding a roller coaster (although there have been some cases of brain aneurisms and the like caused by sudden changes in gravitational forces). Yet, when one rides a roller coaster and begins that terrifyingly slow, rickety ascent that will inevitably be followed by a neck-breakingly fast, perilously abrupt descent, the mind remains unconvinced that no harm can come of the situation. There is a definite fear of dying, whether it be overtly conscious or not. When it comes to horror movies (which are usually a more affordable form of entertainment), the same principle applies, except it's a more vicarious sort of fear, which explains the lower cost. Some hardcore drug users embrace "the fear" that comes from a mixed-drug binge, knowing full well that there are certain physical and psychological risks inherent in their activities.
So with fear comes excitement, and with excitement comes sexual arousal, which explains why amusement parks and movie theaters make such good dating environments. But that's another point entirely, one we will set aside for now.
The main point is, people like being scared, and I like scaring people. Most of the time, when I successfully scare someone (using my preferred stealthy techniques), they shout or scream briefly, and then laugh once they see what's happening. It's not an uncontrollable, giddy laughter, but it still expresses the fact that for a split second they were scared, excited, and now are amused that there were no consequences of their scary moment, save for a slightly accelerated pulse. It's kind of like the relief you get from waking up from a nightmare, when you realize that things aren't as bad as they seemed, and you are now back to reality, where you only have to deal with global warming, cumulatively toxic poisons that you eat and breathe, terrorism, and the like (whew). This writeup is not going to be elaborate or needlessly complicated, as there are probably infinite ways, simple or convoluted, to scare the shit out of someone. I'm going to focus on my favorites, which are quite simple techniques indeed. So, without further ado...
First Method: The sneaky approach and abrupt announcement of presence
This is my hands-down favorite, simply because it's easy, it's abrupt, the opportunities arise often, and it works well. In essense, the goal is to stealthily sneak up behind someone (although depending on the physical layout of the area in which you are scaring them, this may not be necessary) and either shout, scream (or utter some other ghastly noise), or grab them (this depends on your familiarity with the person, and their familiarity with reflex-intensive martial arts like aikido or ninpo tai-jutsu). There are all kinds of variations. For extra effect, you could wield a plastic knife which you will stick to their ribs at the moment of truth. In order to successfully sneak up on someone, you may need to remove your shoes, take your keys out of your pocket, and breathe slow, controlled breaths (skilled breathwork was a very important part of being a successful ninja). A great way to do this easily is to do it in an obviously public place. Say you're at Barnes and Noble with your significant other or friend, and you deviate your course from theirs to go browse on your own. After a certain designated time, you casually sneak up behind them (without appearing too suspicious to onlookers, of course), and grab them. For a split second, you could be anyone else in the store. As far as they're concerned (during their split second of fear), some stranger just decided to grab them for some nefarious reason. Hilarity ensues.
Another example: my mother works nearby and was giving me a ride home one day. She was waiting for me in the parking lot, and I'd just gotten a buzz-cut, which I haven't had for years. I took a slightly convoluted path right up to her driver's side door, which was unlocked. I promptly opened the door and sternly asked her to "get out of the car," which effectively scared the shit out of her. The buzz-cut helped, because she was less likely to notice me as I approached her vehicle in a roundabout way, and because it reinforced the fact that I was "someone else" during that critical split second of fear, in which she thought perhaps that she was being car-jacked in a parking lot. Depending on your vocal abilities, you can do all kinds of fun stuff like impersonate the noise of a xenomorph from Ridley Scott's Alien (my favorite) or scream at the top of your lungs (never done it, probably a bit too much).
Second Method: Temporary Deception
This method could also be described as "be an annoying, lying jerk." But it can be funny, depending on how much of a jerk you already are in general, and how well your friends tolerate such behavior. Let's say your significant other asked you to mail off some critifically important letter / bill / etc that absolutely had to be there the next day. At the end of your workday, as you recline on the sofa, they remember to ask you "did you mail off such and such?" This is where your acting skills come in; your goal is to feign "oops." Again, there are countless variations to this. I don't enjoy this method as much because it's not quite so much "scaring the shit out of someone" as it is "misleading someone to think they are in a bad situation." It's up to you to let them in on the prank when you feel they're ready. With any luck, they might get that adrenaline-fueled sense of relief that I mentioned earlier.
Third Method: Illusions of Doom
I used to work in the kitchen at Chuck E. Cheese, and loved to terrorize (for lack of a more timely word) my good-spirited co-workers using this method. The idea is provide an illusion (involving any combination of the senses) that at least for a moment provokes fear. At Chuck E. Cheese, we handled knives often. Being brash 17 year old males enclosed in an emasculating kitchen environment, we made up for this by throwing knives in the air and catching them, often with negative effects like minor / major cuts and abrasions. It got to the point where we'd see how many times we could get the gigantic pizza-cutter machete from hell thingamabob to rotate in the air before catching it by the handle (I never surpassed one). The other employees were wary of our cavalier knife-handling, which made the prank even easier to pull. Basically, I'd just take a piece of cardboard about the size of a regular kitchen knife, and cut it into the shape of a knife slightly smaller than a real butcher knife. I'd then wrap foil around the "blade," conceal it alongside a real knife in my hands, and seek my target. Once I found a worthy opponent who'd spotted the deadly blade in my hands, I'd gesture as if I were going to throw the knife, taking care to only release the cardboard mock-up. Of course, one must exercise extreme caution, as the last thing you need is to really chuck a blade at someone. Oftentimes, the fear was so real that they'd jump backwards, fall to the ground as it hit them, and quickly check themselves for injuries before realizing what was really going on. I haven't really developed this method much more than this, but your imagination is the only limit to what you can accomplish. Another notable illusion is the Penn and Teller (assuming they made it up; I saw it on a show of theirs) technique of holding one of those liquid coffee creamer capsules up to your eye and bursting it with a fork to create the illusion that you just decided to reveal your vitreous sense of humor to all (tee hee), although I've yet to try this one.
In closing, I'd like to say that you must exercise caution and discretion in choosing your targets. Some people simply don't like being scared. Others (such as the elderly or infirm) might have physiological reasons to avoid being scared. Or, that good-spirited friend of yours might just've had a horrifically bad day, and the last thing they need, no matter how temporary, is another stressful moment to add to the pile. So, do some scouting beforehand, and then let 'er rip if they're a'ripe for a rippin'.
Addendum: this could be "all in my head," but I have noticed that after years of practicing the "first method" mentioned above that I often the scare shit out of people for no apparent reason. I'll just walk in a room and girl will shout "Whoot! You just scared the shit out of me!" It could be that I have simply learned to walk very quietly, or it could be that I run into a lot of timid, jumpy people. Or the most whacked and unlikely explanation is that I have simply developed into a scary mysterious person and my sheer presence alone is enough to freak people out.