I spent some time today deciding how one might divide up the horror genre into bite-sized chunks. Gooder and I tossed around the idea of doing it by the various different styles and waves that the genre has gone through since its beginnings, but I ultimately rejected this because it's a) too hard, and b) not weird enough.
Instead, I decided to spend a bit of time chopping up horror cinema into the various fears they play upon. Never one to start with a whimper, then, we begin the process with considering my own personal favourite horror theme: the apocalyptic clusterfuck.
This particular division can be further broken down. In some horror films the "end of the world" (whether it be the literal destruction of Earth in End Of Days, the destruction of human society in In The Mouth Of Madness, or even the alteration of humanity into something entirely different, as in Shivers) is an ongoing process, something the heroes struggle to stop. The odds may be stacked against them, their chances of success might be slim, but this is Hollywood, and stranger things have happened. There always exists the chance that Armageddon can be averted.
The nature of the fear here is fairly obvious: all things being equal, we'd much rather that our friends and family didn't end up on the business end of a zombie apocalypse. Losing our own lives isn't really a particularly palatable option, but the idea that everyone else is going to cark it too makes things much, much worse. Humanity itself becomes under threat, and once that happens you and I are just tiny cells in a much larger body, statistics that no-one is even likely to look at whilst they try not to get their throats ripped out. If our heroes fail, then we won't have anyone else to turn to. If a serial killer escapes and comes to our neighbourhood, then at least we can call the police, or run for shelter in a friend's house, or what have you. Once we get to this level, though, all help is gone. It no longer becomes about surviving long enough to find a working phone box and dialling 999. There will be no help coming, ever.
Those are the stakes, then. That's what's coming. You can put together a perfectly serviceable horror film that way, though you're going to need either a very strong theme, or a lot of immediate scares as you go, because whilst the ideas I mentioned above are certainly plenty horrifying, as long as the protagonists continue to struggle against the oncoming storm, it's a possibility to be considered, rather than a reality to be faced, and that's not as scary. You can stick in as many maniacs with knives jumping out from behind doors or mutilated corpses showing up in bathtubs as you want, but that's a distraction from The Big Idea, not a reinforcement of it.
The American remake of The Ring is a good example of all this . The Japanese original contains comparatively little in the way of scares for much of the film (it has more important things on its mind). The atmosphere slowly builds to an impressive level, and there are a good number of weird happenings (Ryugi's implicit telepathic powers, for instance, entirely missing in the US version), but actual scares? Not so much. It doesn't need them, the central supernatural mystery and the absolutely terrifying pay-off (Charlie Brooker once said it almost made him shit his head through his arse, and frankly he underplayed it) combined with the human drama (I can't believe I just wrote that) is more than enough to sustain the film. Obviously, this wouldn't do for Hollywood, so any number of totally irrelevant ancillary scares (some cribbed from Dark Water, another Japanese horror film directed by Nakata) were bolted on. Some of them, to be fair, were pretty stylish (the faces of the dead are just hideous), but they still didn't do anything beyond mark time until all was revealed (and, to be fair, the ending is just as horrifying in the remake as the original, though it's worth noting that the original stole it from Videodrome in the first place).
On the other hand, it's entirely possible to chart the progress of the end of humanity in such a way as to make it absolutely terrifying, without having to resort to cheap shocks. Many times, this involves making the apocalypse a gradual process. Often this is done by the tried and tested "Make us into them" idea, which will get a post all to itself, but obviously includes such films as Night Of The Living Dead and The Faculty. Each casualty taken is a new soldier for the other side, which is scary for several reasons, but for now the most important one is that it reinforces the hopelessness of the situation. How do you defeat an enemy that recruits from your own side? More than that, what happens when they recruit your wife, or your child, or your best friend?
There are other ways to intimately link your mid-game scares with the overall picture, of course. One brilliant example of this is Kairo (remade in the States as Pulse, which starred that pretty boy the writers killed off in Lost because he never did anything ever). The film is set in Tokyo, which is becoming both infested with ghosts and simultaneously emptied of people. The ghosts in the film serve a very specific purpose, far beyond just simply appearing at random just as the frantic cellos cut out. The spirits have returned (via the Internet, which might sound stupid but it feeds directly into the overarching theme of people's increasing isolation) to inform humanity that death is "the ultimate loneliness"; that there is nothing beyond the veil of death but an eternity of solitary confinement. Miserable beyond measure at this message, thousands immediately commit suicide (not really the best plan considering what they've learnt, but people can be painfully stupid). You get the best of both worlds (for a specific definition of "best", obviously), since various ghostly hi-jinks are not just designed to scare you at that exact moment, but also tie into the more genera idea that if death is going to be an endless road of total isolation, we should probably think twice about cutting ourselves off in this life too (hence the use of the Internet as ghost-summoner, although anyone who thinks the web prevents you from achieving human contact simply isn't doing it right).
In the second type of ACF horror, though, everything has already gone wrong, and our luckless protagonists are just trying desperately not to get eaten, or infected, or whatever. The best case scenario is now nothing more than living long enough to die of natural causes, which is pretty damn bleak. At this point the fears I described above, of not only being beyond help now but recognising that you will be beyond help forever, come into play. Which is not only scary, but really rather depressing as well, which is a nice one-two punch to the gut that doesn't get used enough in horror. 
The great advantage to this second kind of film is that the mystery has already been "solved". Or, at least, if it hasn't been, then its incomprehensibility has been accepted by the heroes, who are far too busy just staying alive to ask philosophical questions about how they got to this point. Without that mystery, the film-makers have to find themselves something else to fill the gap. There has to be something more than just "They all try not to die, and some of them fail". At least, there should be.
Romero's zombie films are one of the best examples of what I'm talking about here, and Day of the Dead most of all. By this point, the ghouls/dumb-fucks/stenches have taken over the world. They outnumber the survivors by tens of thousands to one. The film concentrates on the various ways the characters (soldiers and scientists, drunks and evangelicals) process the fact that humanity's time is passed.
If anything, the film is the Kubler-Ross model (denial, anger, bargaining, depression, acceptance) applied to celluloid, only the one dying isn't a person, it's a civilisation at best and a species at worst. Logan is denial, convinced that he has a way to keep humanity alive, even though it's obvious to everyone else that it cannot possibly work (how exactly he expects to millions of zombies tamed by constantly feeding them human flesh is a question best left answered, although it does feed my pet theory by being the bargaining part of the equation, "Have this food and don't eat me, please!"). Rhodes and most of his men are anger, blaming the scientists for their predicament and their losses, determined to "blow the piss" out of the enemy even though they know they don't have enough bullets, let alone men, to do the job. Salazar, fairly obviously, is depression, and for much of the film, so is Sarah. She might be more level-headed than anyone else (with the possible exception of Fisher), but she's still massively strung-out, still lamenting the loss of what she personally and humanity in general had, and terrified that what little she has left (including her lover) is about to be lost too.
As the film continues though, she begins to reach acceptance, helped out by William and John, who are already there, and are determined to simply live out their lives finding what comfort they can.
In fact, Day... is an even more poignant example of the accompanying sadness I mentioned before, because it focuses on the exact moment the first type of apocalypse film (the sky is falling!) turns into the second (the sky has fallen, and we ain't getting it back up again). The characters have to make a conscious choice to let go of the hope they've held since the crisis began, and find something else to cling to instead. These films are generally about acceptance to some extent, but what makes Day... unique is that it focuses on acceptance by those who were tasked with affecting change. Viewed from this angle, the fear of losing the support of society becomes instead the fear of being forced to change, to re-write one's internal assumptions and certainties.
Also, you might get eaten by a zombie. Wouldn't want that.
Fears of loneliness and helplessness, of change and hopelessness, and the horror of one day becoming someone who doesn't really mind any of those things anymore. Not bad for movies that are sold on the grounds that ghosts and zombies are going to freak the crap out of you, I would argue. Next time, I'll go into greater detail regarding the fear that we won't just be killed by whatever it is that stalks us in the darkness, but that we will become it, as well.
 I include The Ring in the ACF bracket mainly because the books make it clear that Sadako's ultimate aim is to replace humanity with endless clones of herself. This isn't really followed through in the films, though since the logical extension of the tape's influence is to end up with everyone in the world having either seen it or died from it, I think we can place it in the category of "approaching apocalypse".
 It may say something rather tragic about me that I consider fear and sadness to be so closely interlinked. You can probably add love in there as well, frankly, and thus create the trifecta of intolerable modern life.