Ridley Scott's Alien (1979) and John Carpenter's The Thing (1982) are rightly regarded as masterpieces of the science fiction-horror genre. Both have virtually identical scenarios: a group of rugged people in a harsh, isolated environment battle an unknown organism that has infiltrated their midst. Each has the strongest character assert control over the weaker, more cowardly cast members, and interestingly, both groups use flamethrowers. In fact, those similarities were held against Carpenter when his film came: it was seen as a cynical, gory Alien knockoff released in the wake of Steven Spielberg's E.T. But really, that's a disservice to both movies. While there are some plot similarities, there are enough variations in design and intent to make each experience unique.
For reference, the xenomorph refers to Alien and the thing refers to the various incarnations The Thing.
SPOILERS LIE AHEAD
Alien takes place sometime in the far future on the spaceship the Nostromo. The crew is on a flight back to Earth carrying cargo when they receive a distress signal on an unknown planet. Once there, a crew member is attacked by a parasite attaching itself to his face. Back on board, he seems to recover until a creature bursts through his chest and escapes into the bowels of the ship, leaving the rest of the crew the task of killing it.
The Thing is set an American outpost in Antarctica in winter 1982. After an incident involving a pair of seemingly mad Norwegians and a dog from another camp, the men find the Scandinavian camp in ruins, no survivors. Soon, they learn the dog is no dog; it's a lifeform with the ability to replicate the appearance and behavior of other beings. The men realize some of their own might not be who they say are.
Carpenter has said there are two types of stories about evil: external and internal. The external is the threat of the outsider while the internal is danger from within; the most dangerous being is ourselves. Alien is an external story, and The Thing is an internal story. The crew of the Nostromo must hunt down the xenomorph, and while there is some infighting and a traitor subplot, the main threat remains the creature as picking them off one by one. The crew of the Antarctic outpost must determine who among them is not really human. The enemy has become them, and they begin distrusting and killing each other. Whereas Alien has been called a slasher in space, The Thing is a sci-fi version of an Agatha Christie mystery.
Most interesting about that classification is how Carpenter's film is much more graphic and violent than Scott's. While Alien is not without shocking violence and imagery, The Thing contains decapitations, severed limbs, spilled entrails, autopsies, and plenty of slime and blood. Scott's efforts add to the mystery. This alien can be anywhere and strike at any moment. You're uneasy going around any corner. Carpenter, not unlike David Cronenberg by way of H.P. Lovecraft, displays the fragility of flesh, the weaknesses of the human body, and the breakdown of reality. Expectations, assumptions, and people are literally falling apart.
Set in the future, Alien is beyond the known boundaries of space, and the then-contemporary The Thing takes place in the frozen tundra of the southern most continent. Both achieve an effect of isolation and claustrophobia, and both crews are beyond the reach of help from civilization. In interviews, Scott has stated to the intent was to model the spaceship after the cramped, sweaty conditions of a submarine. The crew of Nostromo, with its course set toward Earth, must destroy the xenomorph before reaching their destination. As the subplot with Ash reveals, their company has plans of capturing the alien and using it for weapons research. The crew are expendable. The thing is already on earth, and if it reaches the mainland, it will only be a matter of time before it takes over everything. The outpost men must find out who's who before spring before a rescue team arrives; by then, they all may be infected. While the Nostromo is far away from our planet and pushing humanity's limits, the scientific outpost is precariously close to home. This gives the crew of the Nostromo hope; there's still time. The scientists and other crew members in Antarctica are already doomed; if someone comes for them, they'll be found the same way the Norwegians were.
Alien has had three sequels, two spin-offs and possibly a couple of prequels on the way. Throughout the film, we follow the creature on its life cycle, from conception to birth to adulthood. In subsequent films, we have become quite accustomed to how it looks and acts, and each film in the series ends on a note of finality: the creature has been vanquished and order restored. Sadly, there is a sense of overexposure in the series. In the first film, the creature is limited to shadows, and we only catch pieces of it visually. But even here, its shape and texture is defined.
Unlike the xenomorph, we never get a clear idea of the thing. We know how it operates, and we see it take many forms, but which is its true form? Because we never stick with one incarnation for too long and it was limited to one film, the thing maintains a sense of the unknown. We know it can plot, strategize, and deceive. Whatever it is, it's an intelligent being. The alien, for all its physical dexterity, strength, and brutality, amounts to an over-sized space bug (note the hive mentality in the sequels). The Thing ends on a note of uncertainty. The camp has been destroyed, and the last time we saw the exposed thing (as opposed to an assimilated form), it was caught in the explosion. But did that kill it? We end with MacCready and Childs waiting to die, but it's entirely possible one is the thing.
Perhaps the most interesting parallel, at least on a thematic level, is the striking sexual subtext. The xenomorph is essentially a rapist. It impregnates its victims, completely violating their bodies to sustain its reproductive life cycle. Note the creature design by H.R. Giger. The chestburster, in particular, as well as various jaws, tentacles, and tendrils are undeniably phallic. It's always drooling, and the final two humans left, Ripley and Lambert, are women. The creature seems to have a sadistic, voyeuristic glee in toying with them. Before it kills Lambert off-screen, we see a peculiar claw-like tail creeping up between her legs. Not only are the victims violated, they are used to breed even more monsters. It's aggression personified.
The thing is also not nearly as aggressive as the xenomorph. In isolation, it will take over a man but will only fight when exposed (Norris' heart attack, the blood test, etc.). The sexual aspect is more subtle. In 1982, attention was just beginning to be paid to AIDS, which was then and still is to a degree today regarded as the "gay disease." Is it coincidence there is not one female character in Carpenter's movie? AIDS is a blood-borne disease, attacking the body's white blood cells. Similarly, on the molecular level, through the blood, the thing infiltrates and assimilates. A man being taken over by the thing is literally having his cells taken over and turned against him. The key sequence occurs when MacReady discovers a way to expose the thing with a blood test. A disease that weakens and rots you from within is not unlike a creature that takes over your body cell-by-cell.
So what can be learned by comparing The Thing and Alien? Two movies with remarkably similar setups can wield wildly different styles. Scott is more deliberate in building his narrative and restrained with the violence. Carpenter jumps right in and features plenty of gore. Both set their characters in harsh, unforgiving environments to confront the terror of the unknown. For Scott, the unknown is a monster with an unmatched viciousness and purpose. For Carpenter, the unknown is ourselves and the beast that lurks within.