Recently on The Groovy Age of Horror, one of my favourite blogs, there was a very in-depth article that discussed at length the dynamic and persistent debate of what is shown in horror and what it means, how it can be significant, and how the medium can make a difference. It's very well-written and beyond my depth to explore here in summary, especially since such depth was reached in the article itself.
However, it did get me thinking, and this is a debate that will go on forever: is it scarier to show something or to suggest it?
I will go with 'suggest', but conditionally. It's more or less universally parroted that it's always scarier to imply things, and that's why popular horror icon H.P. Lovecraft's works enjoy a sort of universal acclaim for being eerie without typically being explicitly filled with exhaustively-described monsters; in Lovecraft's world, the horror is often implied with suspense built up around it, and that is what makes it so unsettling and disturbing.
A quote, perhaps paraphrased, and attributed to Alfred Hitchcock, Val Lewton, and various others, is 'there is nothing more frightening than a closed door'. In the proper context, it's true: there is nothing that is more frightening than the possibility that anything, anything at all, could be behind that door. It's not true in every single situation, of course. A great deal of horror depends on suspense, so if you're not doing horror or suspense as the genre, a closed door won't necessarily be scary. But this is something of a no-brainer.
Sometimes it works, even then. Imagine the audience's surprise when their candy-coated feel-good adventure opens a door to unleash...a horrific villain! It even worked in Star Wars; things didn't feel quite right when they arrived at the cloud city, and the door opening to reveal Darth Vader was genuinely striking and scary. Sometimes seeming dissonance with the ambiance can work very well. A closed door is scary, even if it is also hopeful. But most viewers are going to assume that something complicating or conflicting will lie behind it, because much of drama lies in conflict.
Much of the sense of fear in anything lies in the way a work is realised; if a work is not well-made, it won't be suspenseful. But quality is subjective, and so one person's abject horror will be another person's joke. This is yet another reason why what is shown tends to be subject to how it is shown, and how often. A monster design that is frightening to one person may be ridiculous to another. However, if the monster itself is built up within the narrative of a work and only shown after it has been built up and the audience drawn deep into suspension of disbelief, it will be much more terrifying than something just shown every time it does anything.
It lies squarely in the fundamentals of suspense. If we look at classic horror films, we see suspense as the key, the linchpin to successful tension of the audience. The scariest film I have ever seen in my life is Hush...Hush, Sweet Charlotte. I was so frightened that I got shivers and my skin went cold. But it is psychological horror through and through, and suspense from moment one. There is no monster, nothing is particularly explicit, no grotesque visceral violence...and yet, it is utterly terrifying. It is a descent into madness where the audience is taken along for the ride, where eventually they are made to question everything around them, most of all their own sanity.
Halloween spends the first few minutes showing the audience a painstakingly built-up murder, through the eyes of the murderer. It is not a particularly graphic event, but it is made all the more horrific by the utter revulsion the audience feels -- and revulsion lies at the fundamental basis of much horror -- and the suspense and disorientation that is built around it. The killer, called 'the Shape', is very much the Boogeyman: he can seem to appear and disappear. He cannot be stopped. He cannot be reasoned with. Any motivation that is attributed to his actions is only done by his enemies in an attempt to rationalise it, mostly to themselves. He is barely shown, his actions are not lingered upon...and he is all the more terrifying for it.
The original Alien, while often also classified as science-fiction, is at its root a horror work. Much of that horror lies in disorientation, an inability to understand what the nature of the horror itself is, and why this is happening. Body horror asserts itself strongly, and in many ways, in my opinion, the concept of the self as source of fear is the basis of most fear reactions in horror.
To me, the most horrific thing that an audience can be made to do is to confront themselves. Most people have aspects of themselves that they find distasteful. For many, it is the dark side, the side that could imagine all manner of horrors that they have tried to avoid. The things they would never do, the things they would never say, the situations they hope never to be in...and yet they exist in the mind, and the possibility exists that they might. It is frightening to make the connection in one's own mind, much more frightening. It is easy to laugh or to dismiss a costume that you find silly or uninteresting, but it is much less easy to dismiss what your mind has shown you. In many ways, it is almost an accusation, a call to the audience to confront themselves, throwing open the doors and defences that have been so carefully crafted, to protect them from the aspects of their mind that they fear. They would like to deny that these aspects are there. But, even for a little while, they must confront them and accept their existence. That, to me, is horror.
Even if they aren't aware of what is happening to that level, they're aware that something isn't right. They feel unsettled. They may never understand that a part of the horror they feel is the horror at their own minds and what they can imagine.
This is yet another reason why being too familiar with the monster, antagonist, or what have you is also something that tends to weaken horror. If you become sympathetic to the monster, it is really not as scary anymore. If the monster becomes pitiable, even if it continues doing horrific things, it is more a tragedy than a scary story. Familiarity means that impact of an appearance is lessened.
Every masked slasher film has suffered from this, with some of the worst examples being when the mad killer is turned into some sort of wisecracking antihero that clearly the audience are supposed to cheer for, as he slaughters through the nominal protagonists. They cease being scary, then, and they become a more heroic figure, even if what they do isn't what most people would consider 'heroic'; they are champions of a sort, for the audience.
Even extreme acts of violence can become pedestrian if there is nothing to build up to their horror. Having explicit acts of visceral violence through a film is often nothing but a masturbatory gesture, and it builds up audience insensitivity. Just as in works where the world presented is thoroughly unsympathetic along with all of the characters, that acts against the whole point of horror. If you don't care, you won't be scared. If you aren't appalled by what you're seeing, or what's being implied, then you won't experience horror.
Similarly, it is easy to point out the differences between Alien and Aliens. The first is a horror, the second is a science-fiction action film. While there are suspenseful parts in Aliens and horrifying parts -- the latter of which are mainly when the audience considers the ramifications of what has happened in the colony, rather than anything being shown -- largely it is an action film. The creatures are not as terrifying as the original in Alien, both because there are so many of them (thus robbing it of its uniqueness and making it more understandable due to social context), and because they are seen much, much more frequently and clearly, as well as destroyed with far less effort and trouble than in the original film.
The Blair Witch Project was one of the most successful examples of horror implied rather than seen, and it was also among the most well-received...at least initially. However, largely due to social saturation and everyone knowing the film's secrets, people weren't as scared, the longer it was out. This isn't surprising; if you know you're not going to see a 'Blair Witch' going into it, you don't get the suspense up that it will happen. It's another case of the horror being unveiled and too familiar to the audience, rather than the suspense built up and the secrets kept, and it illustrates my point all too well: familiarity breeds comfort, which is the antithesis of suspense, tension, and horror.
It doesn't always work, I'll be the first to admit. Monster films tend to be the worst of them, because either they have a costume they know is goofy and just use the technique to hide, or they don't have a monster at all and try to build suspense in the hopes that no-one will want to actually ask the important question. Usually, there has to be some kind of horror present, even if only strongly implied. The most terrifying situations are ones that involve the audience, even vicariously.
The Cask of Amontillado is terrifying because of its situation, and the fact that the audience has come to identify with the victim. They have been brought into that world. The Whisperer in Darkness, similarly, builds up a suspenseful mystery that is made into horror in the last few sentences, when the reader realises that (through the protagonist's narration) they themselves were so close to that unspeakable horror and didn't even know it at the time. It's the same with stories like Pickman's Model.
When you get right down to it, it can just be said that suspense of the unseen is also easier to do relatively well, whereas suspense of the clearly seen is not. It takes great talent and great vision to have something seen enough to become familiar and still retain its feeling of terror. It does not take so much of that to make something barely-seen or unseen scary, because the audience has no way of concretely saying 'this isn't scary, and here's why'.
Being explicit can also disrupt the ambiance of a story. If a story is meant to be more lighthearted, action-oriented, or otherwise having a completely different mood built up from the beginning, throwing in something grotesque or explicitly horrific can completely disrupt the mood and create a dissonance that damages the work. It is one of the reasons why horror-comedy combinations tend to be either loved or hated passionately, and why the genre is so small; humour and horror are difficult to combine successfully because their moods are so discordant. Implications of horror work better in these cases since the full gravity of the situation may be delayed until a point in the story where it can be realised with more impact.
Ultimately, the debate will continue ever on. However, it is my opinion that implied, subtle horror tends to work better in most cases than more explicit, visually-shown horror. From practical considerations such as budgets and writer ability, to more profound considerations such as the confrontation of the self, I feel that the suggestion of horror is usually superior.
Your opinions may vary.