Wanting something without much of a plot that I could let play in the background while finishing up my paper, I put on Saw. I dislike any kind of shock-driven horror, and haven't seen any of the so called "torture porn" films (although Takashi Miike's The Audition would be close - and that one scarred me, deeply).
Like so many films in the past decade or so, Saw's narrative comes in part from a video game structure - it's all about succeeding within the rules of a game and graduating to the next level, with an eventual goal of "beating the game." These films achieve whatever resonance they have by playing with or destabilizing these rules - the world of the characters is first shrunk to the parameters of whatever game they're supposed to be playing, then it's revealed that those rules, which would be the one dependable element of their existence, are in fact not dependable.
What's interesting about the film from a cinematic standpoint is the lengths it goes to in order to mobilize the traditional cinematic horror devices. The horror comes from the same place it has in any film since Psycho or Halloween - the film depends on its ability to maintain its villain's status as being essentially disembodied: the separation of voice from body and, when the body is shown, giving it as few human characteristics as possible (particularly a mouth). When Danny Glover first encounters the killer, he has no chance of controlling him, because he has no human vulnerability or identity. At this point in the film, he's basically supernatural. He starts to lose his invulnerability little by little: first we see his fingers, then we see a little bit of his face (and probably figure out who it is), then his identity is revealed.
A man with a rifle in a well-lit room might be more certain to kill you in an attack than a skinny psychopath lurking in the shadows with a power tool, but horror films mobilize in the audience an irrational fear of the unknown and incomprehensible. You saw it in (again) Psycho and in Halloween, villains who have no clear origin for their monstrosity. With Psycho, however, the villain is human, sort of, and vulnerable as soon as his/her true face is shown. Michael Myers's face is never shown, and he never becomes vulnerable - what causes the monster's psychological drive to kill or his inhuman invulnerability is never explained. He never becomes human, or comprehensible in any physical or psychological way, and therefore the characters' attempts to fight, cure, reason, etc. are at best temporary solutions. Michel Chion's writings about the "acousmetre" - the bodiless voice - are essential to understanding the mechanics of the horror film. In a horror film, particularly a slasher film, the filmmakers go to ridiculous extremes to not only hide the villain's face, but to maintain a disjunction between the voice and the mouth from which it issues. Audio distortion (since Scream?) is a favorite, so are masks, phones ("I don't want to alarm you, but the call is coming from INSIDE THE HOUSE!"), shadowy cloaks and robes. Or how about all of them, plus a doll, in the case of the Saw films.
Roger Ebert, in his review of Saw, writes this: "As for the (possible) Jigsaw Killer, he of course is glimpsed imperfectly in some kind of a techno-torture lair, doing obscure things to control or observe the events he has so painstakingly fabricated. We also see another version of the killer, also annoying: Jigsaw (or someone) disguises himself as a grotesque clown-like doll on a tricycle. Uh, huh. Whenever a movie shows me obscure, partial, oblique, fragmented shots of a murderous mastermind, or gives him a mask, I ask myself -- why? Since the camera is right there in the lair, why not just show us his face? The answer of course is that he is deliberately obscured because he's being saved up for the big revelatory climax at the end."
It's this irrationality - both in the threat and in the structure of the narrative - that separates what we think of as "horror" films from suspense films or thrillers. I know very little about the history of the genre, but this seems to be a recent development. In Hitchcock's day, it wasn't unheard of to refer to his films as horror films. The giant monster or alien invasion films of the fifties - what we would consider strict science-fiction - were also horror films, sort of. If the Universal horror films (Frankenstein, Dracula, The Invisible Man) are still considered "horror" today, it's because their monsters are human-like but still inhuman, and are supernatural in that rational methods of defense against them don't work, at least initially. (Viewed today, James Whale's films are more comedies or melodramas than horror, or even science fiction - but the monsters of the Universal films are so iconic that when they are considered in cultural terms today, the tragic figures from which they've descended are less important than the subsequent iconography.)
The irrationality of these horror villains is constantly being thematized in these films. You see it again and again, victims being put in a perilous situation and trying to work their way out of it by figuring out the exact nature of the threat. Once the threat is supposedly decoded, the villain is suddenly less supernatural and seemingly vulnerable - but the last big twist of these films is usually something that reveals these efforts to be in vain. Naomi Watts figures out the little girl's story and thinks she's discovered out how to end the cycle of supernatural violence, the action lulls and then all of a sudden it comes roaring back as she and the audience realize that no one understands anyhing. That is, in The Ring as in Saw, the audience is placed in the same position of trying to translate an irrational, supernatural threat into something rational and concrete that can be dealt with. In Zizekian-Lacanian terms, horror films are scary because they traffic in this irreducible kernel of irrationality, which can't be verbalized or reasoned away. It's an irrational kernel within our own make-up (Zizek would say, and maybe he does somewhere or other), one that is simultaneously within us and not part of our reasoning ego - both an internal and external threat.
The Saw films, with drastically diminishing returns, become less like horror films as the series progresses and the killer becomes more prominent and more humanized. Their ridiculousness, and their moral offensiveness, comes from the way the films humanize Jigsaw and turn him into a moral authority - that is, when the killer's face is revealed, when his voice is wedded to his mouth, he no longer becomes a source of horror, danger, violence. The film's can't conceive of that irreducible irrational threat as existing within a flesh and blood human being, and so it displaces the responsibility for the crimes onto the victims. The horrific irony is that the victims become less human, more caricature, as "Jigsaw" becomes more human.