In many ways film is about spectacle, and in no area is that clearer than in its many ways of treating the human body. The history of putting a body on display for the viewer goes back a long way, long before cinema. It goes at least as far back as the "strong man" acts of 19th century circus shows, which would feature a large musclebound man performing "unusual" feats of strength to amaze the viewers. The earliest cinematic example I have found is one of Edison's Kinetoscope films, Sandow the Strong Man. The display of a muscular body (usually male, though female examples are becoming more common) would become an important element of the action film, and the same basic principle is still present in many contemporary works (Taylor Lautner in the Twilight series, for instance).
However, there are other ways to create a spectacle using a human body of either gender. On one end, there is the display of an impressive muscular build, and the display of the body in motion. On the other, there is precisely the opposite, something much darker and more disturbing. Instead of contriving situations to show off the body, situations are instead developed to show its destruction. The appropriately-named The Wizard Of Gore emphasized this very idea. The titular character, Montage the Magnificient, would create "illusions of illusions".
In one scene he performs an inversion of the classic "sawing a lady in half trick" wherein he actually saws a female audience member in half (with a chainsaw) but makes it appear to be an illusion. In the same scene, Montag delivers a speech in which he explains that "terror and torture have always fascinated mankind". He likens it to the pleasure experienced by spectators of Gladiatorial shows in Ancient Rome, which while perhaps not as violent as often depicted in the movies (the fights weren't always to the death) was still based around people harming each other to entertain others.
Indeed, there does seem to be a sort of sick, twisted pleasure to be found in observing the abuse inflicted on another person's body. This makes up an entire sub-genre of horror commonly known as "body horror". The idea behind it is simple, to frighten the viewer by mutilating or distorting the image of the human body. When done right it can be a very effective tool for horror, but in a strange sort of way it also serves precisely the opposite. The abuse or distortion of the body becomes less a mere source of fear and becomes itself a source of spectacle.
The Wizard of Gore gives the viewer a unique view compared to the fictional audience within the film, in that we get to see the violence in its excruciating detail, while we also see the perspective of the film's audience (that watches oblivious to what is really happening). When the viewer watches a body (often female, for some reason) being mutilated in this fashion, it creates a bodily response of sorts, much like watching a martial arts movie.
When watching a movie like Enter the Dragon, there is a feeling that the viewer experiences. When they watch Bruce Lee beating up one of the antagonists, there is a feeling of tension that excites them. It makes the viewer feel as though he or she is part of the action, right in there next to Bruce taking out the bad guys. This concept is known as "muscular sympathy", and has a major role in the popularity of action films featuring martial arts, though it does appear in other forms of action movies as well.
Just as martial arts films rely on the spectacle of bodies in motion, the horror genre relies on the spectacle of bodily abuse. The same basic principal of muscular sympathy applies to the horror genre as well, if in a different form. In a similar vein, the viewer is excited by a tense feeling (which I shall refer to as "muscular repulsion") that makes them feel like they are experiencing what is on screen. The difference is in precisely how the viewer feels. Instead of feeling like they are right next to Bruce Lee beating people up, it becomes more as though they are the ones taking a beating at his hands. More accurately, the viewer feels as though they are experiencing the pain of whatever horror is being inflicted.
A perfect example would be David Cronenberg's The Fly, in which Jeff Goldblum's character, Seth Brundle, accidentally has his DNA merged with that of a fly. What makes this particular film so terrifying is watching him slowly transform into something that seems less human. We see the process over several weeks, perhaps months, as he slowly transforms into a disturbing hybrid. As he does, his body becomes distorted, and it becomes increasingly harder to tell where he ends and the fly begins. When watching The Fly, the viewer experiences a sense of pain, because they feel as though they are experiencing the transformation as well. It seems as though the viewer knows what it feels like to be transformed in this way.
This feeling is also especially prominent towards the end, when the final stages of the transformation are taking place. First there is a spectacle of the harm inflicted on a supporting character, Stathis Borans (John Getz), who loses his hand to the titular monster. In this moment, the viewer feels as though they too are experiencing the same pain (and of an especially excruciating sort, seeing as his hand appears to be slowly corroded off his arm). Later on, when Brundle (or "Brundlefly" as he is often known at this point) is merged with the teleporter itself, he crawls out in excruciating agony. The viewer once again gets a sense of the horrendous pain that comes from his final transformation. Just like in the martial arts film, it feels as though they are part of the action.
Muscular repulsion is certainly a major part of the experience in watching a horror film of any sort, especially one with large amounts of gore. Just muscular sympathy makes the viewer feel like they are fighting alongside the hero, muscular repulsion makes the viewer feel as though they are on the wrong end of the action. It is a major part of most horror films, especially ones that feature large amounts of gore. In a similar vein, The Thing features the same experience of muscular repulsion in specific moments.
The outdoor scenes make the viewer feel as though they are in the freezing wasteland of Antarctica, while indoors they get the same sense of paranoia, but it is in a few select moments when moments of true muscular repulsion happens. These include scenes such as the infamous scene where Copper (Richard Dysart) attempts to use a defibrillator on his fallen colleague Norris (Charles Hallahan), only to discover the man in question was infected. Nobody (including the viewer) realizes this until Norris's chest opens and slices Copper's arms off, leaving him to die presumably from a combination of shock, blood loss, and smoke inhalation.
Like in The Fly, the viewer is excited by a bodily response in that they seem to feel the same pain as Copper. They experience a sense of shock and experience muscular repulsion in that they feel as though they have just been through the same experience. This is one of the best examples, but muscular repulsion is present at numerous other moments, such as when Bennings (Pater Maloney) is assimilated, and Windows (Thomas G. Waites) is killed by the Palmer-thing. The simplest case arguably happens just prior to the blood test scene, when Clark (Richard Masur) is shot in the head by MacReady (Kurt Russell), when the viewer feels as though they too are on the wrong end of revolver he is holding at the time.
The Thing also provides another method of using bodily abuse as a spectacle, in the form of distorting and transforming the body. In this case, it happens when the characters who have been infected are exposed. With each one, they begin to transform, usually still somewhat resembling the victims but in a mutilated form. During the scene where MacReady performs the blood test, Palmer (David Clennon) is exposed as a Thing and immediately his body begins contorting. His face becomes elongated, his skin shrivels, and turns into a sickly reddish-brown color that almost looks as though he is decomposing. Once the Palmer-Thing breaks loose of its restraints, its head splits in half and turns into a mouth.
Similar approaches happen with the other cast members that are exposed as well. Norris opens his chest like a mouth full of sharp teeth and then proceeds to disconnect his own head (which then drags itself along the floor with an elongated tongue before growing legs and a separate pair of eyes), while the body grows a new one out of the torso (that resembles Norris with fangs). When the Blair-Thing transforms in the film's climax, only half of his face still remains visible (attached to the back of a sideways head filled with sharp teeth). Even Bennings, who is infected much earlier in the film, is shown first in the process of assimilation (his body is coated in a sickly red and wrapped in tentacles), and then partially assimilated just enough that the viewer can recognize him while also seeing clearly that he is no longer human.
At its core, the principle is basically the same as in the action movie. Much the hardbodied heroes of the 1980's or even Sandow the Strongman, the human body is displayed as a spectacle for the viewer to admire. The difference is in how. Much like how a film such as Predator impresses the viewer by displaying the extremely muscular bodies of its actors, movies like The Wizard of Gore, The Thing, and The Fly attempt to impress the viewer by doing the exact opposite. Instead of displaying a muscular build, they display the body being destroyed, mutilated, distorted, or in some other way being abused. This in turn becomes a major part of what makes a lot of horror films work, and how they affect their viewers.