You know, I firmly believe that at the heart of many – possibly all – genres there is a single sentence. There is certainly one at the heart of all Horror movies, and I’ll tell you what it is in a moment, but first I have to go into a few things to make it clear.

The most important thing to understand about genre is that it’s all about emotion. Comedy is supposed to make you laugh (though you might not believe that if you saw some of the scripts I see). Thrillers are supposed to scare you. And Horror – is horrifying. Horror movies contain horrible images, sounds and ideas. We get to be afraid too, often, but it’s horror that is at the very core of every good Horror movie.

Horror movies have dismembered bodies, scary ghosts, dark corners where things lurk that we cannot bear to think about. We meet sociable cannibals with gourmet tastes in human offal, murderous dead mothers who have taken over their sons’ psyches. We get incarcerated by pleasant serial killers, turned into monster flies and find our very bodies and minds turning against ourselves. It is the duty of the Horror writer to introduce us to nightmares we never even knew we had.

Now, here’s the thing: we want to be horrified, and yet we don’t. Unpleasant feelings are difficult. We both want to feel them and at the same time we shy away from them. All our natural instincts tell us to run from anything horrific.

The second job of every good screenwriter is to ensure that we can’t run. When we hear the floorboards creak, or see the tell-tale dribble of the Alien, or feel the chill of the mist around the graveyard at night, the writer must shut the door, immobilise the car, ensure the mobiles don’t work and that we have nowhere else to go than where he wants us to.

So the sentence that every great Horror writer has engraved on their heart is: Don’t go into the attic!

Every Horror movie has an attic – real or metaphorical – where we know horrible things live. That “attic” might be an apparently empty spaceship, or a Californian motel, or even a sunlit motorway service station. The writer must build up exactly why we don’t want to go there – and then make sure that there is no possible way not to.

This is also why credibility is so important in Horror scripts and why in my Horror and Noir workshops I always include a section on credibility – the skill of getting an audience to believe your story at every step along the way. All the way into the attic.

There is also a core sentence in Noir. I’ll leave you to work that one out for yourself – and I’ll give you a clue. As Noir grew out of a dark marriage of Horror and Crime so the core Noir sentence is related to the Horror sentence.

And if you want to learn more about Horror and Noir, come along to the Euroscript Horror and Noir Evening in London – 6pm on Tuesday June 29. Click here for more details. There are currently four places left, and it’s the only time it’s on this year.