A horror story might be defined as a story in which the primary intention is to scare the audience. Being scared (within the confines of a safe environment) is fun. People spend money to watch horror movies and ride roller coasters for roughly the same reasons.
In the same way a comedy needs to be funny in order to be successful, a horror movie needs to be scary. Scaring the audience isn’t just the prime goal, it’s the film’s raison d’etre. A movie that isn’t at least trying to scare the audience isn’t a horror movie; it’s something else.
However, I have a theory, which is this: Horror is most successful when the characters and set-up are such that the horror element (ghost, demon, monster, killer, etc.) could be removed and we would still have a story to tell.
By way of example, look at THE SHINING. Let’s say the Overlook Hotel wasn’t haunted. Without ghosts around to bedevil the Torrance family, we would still have a drama about Jack struggling with alcoholism and failure, using this remote caretaker job to reflect on his actions and find a way to redeem himself with Wendy and Danny.
Or THE EXORCIST. If Pazuzu stayed out of it, we would still have a story about a recently divorced movie star struggling to balance her career and motherhood.
Again, we’re talking about a drama, but that’s only because both THE SHINING and THE EXORCIST are a) based on novels that give a lot of juice to b) well-developed characters dealing with universal themes revolving around family.
But we might apply this same approach to classic slashers like HALLOWEEN and FRIDAY THE 13th. Take away the psycho killers, and we’re left with an ensemble of characters trying to get drunk and laid, i.e. the standard business of teen comedies.
Or let’s say we have the story of a normal guy who takes in a rough man with a dark past who repays the hospitality by seducing his host’s wife so powerfully that she’s willing to engage in murder to keep him. If we introduce a horror element like the demonic Cenobites, we have the original HELLRAISER. Take them away, and we have the noir thriller THE POSTMAN ALWAYS RINGS TWICE.
Which isn’t to insist that this theory absolutely applies to every horror movie ever made 100% of the time. But in writing these things, we can and should look at the story through this prism.
Because without the crutch of the horror element to provide “value,” we then have to put effort into developing the characters and their story. The audience can better engage with well-developed characters. Now the audience cares about them when they are endangered, and thus the horror elements actually work; they have emotional affect.
Without that development and emotional engagement, all we’re left with is shouting “Boo!” at the audience. Which works in the moment, perhaps a couple of times, but isn’t enough to carry us through a feature-length narrative.