"I have excellent news for the world: There is no such thing as New Wave.
It does not exist.
It's a figment of lame cunts' imagination, there was never any such thing as New Wave, it was the polite thing to say when you were trying to explain that you were not into the boring old rock and roll but you didn't dare to say punk because you were afraid to get kicked out of the fucking party and they wouldn't give you coke anymore...New Wave doesn't mean shit."
-Claude "Kickboy Face" Bessy
So here's a thought exercise.
Picture a movie in which people are disemboweled and hung from cages, faces are cut off and worn like masks, people dance around in wigs made from human hair and scalps, and women are kept in pits in preparation for being starved, murdered, and skinned.
Straight-to-DVD gorefest? No, it's a thriller which won a bunch of Academy Awards.
How about a movie in which people are forced to eat until their stomachs rupture, wired to beds until they wither away from neglect, made to cut hunks of flesh from themselves and weigh them, fucked to death with bladed dildos, and mailed their wives' severed heads?
Cheap torture porn exploitation trash? Nope, critically acclaimed thriller.
Hmmm. Okay, how about a movie in which a couple of people are kidnapped by a doomsday cult and struggle to escape?
Taut, suspenseful thriller? Nope, straight-to-DVD horror.
Well, crap. How about a movie in which a group of college students run afoul of some criminals in Moscow and have to escape their clutches, despite corrupt and ineffectual police?
Political thriller? No, low-budget indie horror.
So then what distinguishes a horror movie from a thriller?
I am of the opinion that although there are some characteristics which traditionally separate thrillers from horror movies, these distinctions have become less definitive of each type of movie over time, and as a result, this distinction is now more semantic and rooted in cultural expectations than anything else.
Seems to be a pretty handy way to segregate types of entertainment and maintain some sort of classist status quo as well, but I don't blame you if you're rolling your eyes at that bit.
So where to start? Where do the distinctions hold up, and where do they fall down? I'm going to talk about these two types of movies in terms of three things - what they're about, what responses they evoke, and who they are for.
Let's start with what they're about. Horror movies are customarily the province of the supernatural - ghosts, monsters, possessed objects, fantastic beasts, demons, and other assorted agents of the great beyond. There's some sort of threat, and it's a threat that a conventional, reasonable view of the world would or could not predict. The irrationality of the threat even figures in in many cases, as the threat is multiplied by the unwillingness of other people to believe that it is indeed real. The supernatural nature of the threat can also be extended to the extraterrestrial (e.g., Alien) or an otherwise natural agent with an apparently supernatural resistance to harm (e.g., Halloween, Friday the 13th) or supernatural abilities (e.g., Carrie). What's important is that the threat isn't realistic.
Conversely, thrillers deal with realistic threats. Spies, criminals, killers, etc. The exact capabilities of these assorted bad guys might be exaggerated for dramatic effect, the plots may be unrealistically complex, but unlike ghosts and werewolves and devils, spies, criminals, and murderers exist. There are still people in jeopardy, and the jeopardy (as in horror movies) may be on anything from an intimately personal to global scale, and the film's protagonists may also expect doubt and resistance from other people, but it's based on "your situation is highly unlikely", not "the threat you say exists cannot possibly exist."
The problem with this distinction is that increasingly, realistic threats are the centerpiece of horror movies as well - Hostel is at least as plausible as any spy thriller, the first Saw film stretches plausibility a little, but there's nothing supernatural about the traps Jigsaw constructs. Even going back to the late 60s, Last House on the Left is not at all supernatural. But none of these movies are generally thought of as thrillers. For that matter, some thrillers stretch the ability of their antagonists into almost supernatural territory. Serial killers, in reality, are not omnipotent criminal masterminds like Hannibal Lecter or John Doe. They don't set up elaborate tableaux like Doe does in Seven or like the killer in Anamorph. To make things even more complicated, one of the most realistic portrayals of a serial killer - Henry: Portrait of a Serial Killer - gets talked about more in horror terms than thriller ones. So what the movie is about isn't a reliable distinction.
So what type of response does each movie evoke? Hell, that's right in the names to start. Horror movies presumably are meant to inspire horror in viewers, and thrillers are meant to inspire thrills. There's a little bit more of a positive connotation to "thrills" than "horror" - thrills can also imply excitement. Horror doesn't really have a positive connotation. But either way, there's a sense of rising tension and fear associated with a threat to the film's protagonists, which could also be described as dread. Whether it's Jason Bourne eluding assassins or Laurie Strode eluding Michael Myers, we simultaneously root for escape and dread finding out what failure to escape will bring.
Now, arguably, we don't always root for the protagonists in some horror movies. Maybe we are watching otherwise unsympathetic characters get slaughtered and enjoying it. But there are also revenge films where an antihero goes after the people who might otherwise be the good guys - I'm hard-pressed to find a big distinction between a bunch of callow asshole teenagers getting picked off in a horror movie like Shrooms and callous asshole cops getting picked off in First Blood. So either way, the audience is presented with an opportunity to watch what is generally a mortal struggle and invited to respond to the life-and-death events with the appropriate emotions, knowing that what we're watching is fiction, making our experience of these emotions safer than if we found ourselves in the position of the protagonists. I'm hard-pressed to find many distinctions between the two types of movies based on our response to them.
So finally, for whom are these movies made? This is where my introductory quote (from the fantastic documentary The Decline of Western Civilization) comes in. I began by saying the difference between the two types of movie is primarily semantic, but semantics are important. What we call something helps to determine what place it occupies in our culture. I already touched on this a little with the implications of calling certain types of horror movies "torture porn", which is probably apropos considering one of my favorite examples of semantic distinctions is that between erotica (whatever I like) and pornography (whatever you like).
Horror, as a genre, often carries a stigma. The term "horror fan" implies that one is a consumer of horror as entertainment (as opposed to appreciator of horror as art), and so by extension that one is entertained by the things that happen in horror movies. Since what happens in horror movies is often, well, horrifying, this suggests something unsavory about the person, and the idea of entertainment (specifically as opposed to art) also carries class connotations, to the extent that art is associated with the elite and entertainment is associated with the masses. This understanding of "horror fan" then is one of someone low-class who delights in unpleasant things. That appeals to horror fandom often hinge on some variant of this interpretation, ironic or not, doesn't help things any. Horror movies don't win awards, and are rarely acclaimed by mainstream critics.
On the other hand, thrillers often contain similar content and evoke similar feelings, but don't share the same negative connotations. Thrillers may not be appreciated as art either (and any distinctions between horror film and art - or whether there should be any or not - is yet again a whole other post), but I'd argue there's no cultural construction of the "thriller fan" that relegates them to some sort of unsavory cretin class. At worst, thrillers are a guilty pleasure of the elite. They push the same buttons as horror films, but without the same specific stigma. Call it the difference between cocaine and crack.
So, then, maybe it's the degree to which cultural elements approved of by an elite - specific actors, directors, etc. - are involved in a movie that might in some cases get them labeled as thrillers. All of the stuff that happens in The Silence of the Lambs or The Sixth Sense is straight-up horror shit. But Jonathan Demme doesn't make horror movies. Jodie Foster and Anthony Hopkins don't star in horror movies. Bruce Willis doesn't make horror movies. So these are thrillers instead. It's the polite thing to say when you are trying to explain that you like movies that scare, upset, or disturb you but you don't dare to say horror movies because you are afraid to get kicked out of the fucking party.