Don't look outside your window. Not at the moment. Not while you're sat safely inside. Outside is darkness, punctuated by streetlights and shadows. And right now, while you're comfortable, lit by the brightness of the monitor, you don't want to introduce doubt into your little bubble of light and safety.

Because if you look outside, the Slender Man might be there.

He might be stood by the trees or by the streetlight, with what can only be described as his face looking up at you. That height and those limbs just looking wrong, and that alien intensity. And you don't need that right now.

We're talking about a fairly specific kind of horror here. The Alternate Reality Game horror. The kind the works on the basis that 'it's all real, honest gov'. It all started with the Blair Witch Project. Well, no it didn't, but the current crop did. What it started with was the old cliche of 'last written evidence' stories.

These were the kind of stories that have been adapted into every media. The kind of story that were someone's diary, or a letter, with the clues to what happened to them strewn through the narrative, and invariably ending with their deaths. Quite often, for added 'truthiness' (Copyright Stephen Colbert), the stories would end mid-sentence. 'And I'm so sared and I can hear him at the door and oh God, he's breaking the door down and he'll be here in a '. The format of the stories provide a large part of the thrill and scares.

The basic plot tends to be fairly simple. Something happened to somebody, and their records of the event form the story. Quite often there'll be the framing device in which we're being told the story by someone who is researching the records, in order to find out the truth behind what happened. Of course, while researching the records, they unintentionally retrace the missing person's steps, putting them in danger of the same outcome.

So what makes 'The Blair Witch project' so important? It added the final ingrediant - the internet. By adding in a slight element of interactivity, it makes the frights a lot simpler to achieve and also more effective.

Here's how it works.

You, the viewer, are provided with the clues in video format. The people who made the video footage have gone missing, and the footage has clues in it which explain what happened. By viewing it, you become aware of some of the subtle signs and oddities that accompany the horror. Stick men and children's handprints become an important part of the story.

But there's nobody else putting these things together. You see, you're smarter than the people in the footage. They don't know what the end-point is. You do. You already know that something has happened while they filmed that caused them to go missing (presumed dead). So you place more importance on these details. After all, you know full well that there's something out there, so that voodoo doll is less likely to be a practical joke and far more likely to be something very important.

So you're not looking for what they were filming. You're looking for what they accidentally picked up in the background of what they filmed. You're looking for the pieces that make sense of the whole picture - and unlike the poor victims in what you're watching, you actually have a sense of what the whole picture is.

Now, this interactivity is absolutely key. The more you can make the viewer feel like part of the experience, the more likely the piece is to work. So, when 'The Blair Witch Project' was put together, it was done with a brilliantly viral marketing campain. People were looking into this and buying into the mythology behind it. After all, the film-makers obviously knew what was actually going on. So it could be worked out. And perhaps it was more elaborate than was originally believed.

And the team behind the movie played a blinder. Drip-feeding information to the internet, and allowing the ever-growing army of fans to drive themselves into a frenzy, and all of them getting freaked out because they were slowly becoming aware that maybe delving too deeply into what had happened was a bad idea.

It's a bit like going back and watching the BBC's most terrifying piece of drama, 'Ghostwatch'. Made in 1992, it was done in the style of a live show which was keeping tabs on a 'haunted house'. Rather smartly, it was a normal semi-detached house rather than an old mansion, with a normal family in. The background story was within the previous forty years. It all felt so much more plausible.

And then came the fantastic moment. During one scene, there is someone standing by a set of curtains that shouldnt be there. It's not obvious - it's in the background. Not everybody would have caught it, but a lot did. And they phoned in to the live phonelines (which were the same as 'Going Live', meaning that '0181 811 8181' is burned into my memory), leading to the studio host rewinding the footage - and nothing was there.

For the first time I can remember, a scare from a horror story was effective because you, the viewer, were the only person to see what had happened. You were part of the story, and the fright was gained from the idea that nobody else had seen it - just you. It was, to put it bluntly, fucking terrifying.

Yes, it was a drama. It had been given away, sadly, by the Radio Times, who published a cast list. But it didn't matter, since it was a stunning piece of television. Unfortunately, the director and writer were stopped from doing something they really wanted to do, which was having an ultrasonic sound emitted during the broadcast at a point where it was made clear that pets would react if supernatural activity was occurring. Actually, there's nothing unfortunate about that at all - that might have genuinely traumatised some people. But God, it's a brilliant idea.

That's where the 'Alternate Reality Game' aspect comes into it - by viewing the story, you become part of the story.

And, of course, you might be the next victim. That's the real key to scaring the crap out of your reader/listener/viewer. Making them think that, not only is this all real, but that they're part of it.

And that's where the internet comes in.

Take a look through these pictures, retrieved from a lost camera, ideally before reading the next paragraph. And look carefully.

How quickly did you spot the element that turns it from a series of odd pictures into something quite frightening? How easily did a series of pictures become a story, and one that isn't very pleasant? And how does it work?

These were created by a poster on the Something Awful forums as a piece of fiction*. Originally containing more overt elements, it was suggested that it worked better when it was more subtle. And this is undoubtedly true.

My belief as to why it works is that it relies on the point that the person who created it didn't notice the disturbing elements. The hoax has been replicated word for word on various forums, and the key to it is that it is other readers that notice something has gone on. And when you notice something in it, you become, effectively, the first person to notice this. There's no context or explanation to soften the blow. Instead, you have to work out for yourself if this is something real or not, and if you're the person that's noticed the moment that some interesting photos become evidence in a murder investigation.

This element of interactivity can override the knowledge that you're involved in a fiction, allowing you to immerse yourself intentionally in the game more. It was used to strong effect in the early part of the viral campaign for 'The Dark Knight'. Everyone knew it was a game, but played along - the playing along allowing people to become more emotionally involved, in the same way as the missing camera story.

This leads to the Slender Man and Marble Hornets.

Marble Hornets is a story being told via Youtube uploads. The basic set-up is classic. A film student abandons the project he is working on after acting strangely, and then goes missing. A friend goes through the raw footage and uploads anything that may help explain what happened.

The excerpts of footage range from thirty seconds through to eight minutes or so, and it is important to pay attention at all times. As it progressively becomes more strange, it also becomes more elaborate. The narrator has an in-character twitter and facebook, and has responded to questions. He's updated his search as it has gone on, and the resultant mess he's found himself in.

To explain much more would spoil it - this is ongoing, and well worth taking the time to watch. It's low budget, not particularly well acted, and far from professional, but it is very, very clever.

And it involves the Slender Man.

The Slender Man is, again, from the Something Awful forums. Created in a thread named 'Create your own paranormal photographs' by a poster called Victor Surge, he quickly became a highlight. Added into the background of two photos of groups of children, supposedly in 'file photographs' of tragedies, the Slender Man was a tall man with no specific facial features, whose arms were slightly tentacley.

He would hide away, and once you saw him, he would entrance you towards him. It would only be when you came up close that you would realise how inhuman and horrible he was. Obviously, it was key that we never saw this in the pictures. In the pictures, he was always indistinct, just hovering on the edge of our perception.

It taps into very primal fears - the idea that you may look out of your window, see nothing wrong, and then look again, and see that he's been there all the time. Watching you with that awful non-face.

It worked so well that the thread got totally overtaken with the Slender Man, with people going to lengths to create their own versions of him, and their own stories of people's meetings with him. A backstory dates back to lutheran times.

The point where it becomes the prime example of what we're talking about is when the people involved in creating the myth start getting scared by it. It defines the idea that maybe there is more to this than we think, if we just look closely.

*I'm not intentionally avoiding giving due credit - I haven't been able to find the original thread. If anyone can supply me with the correct credit, I will happily edit it in.