Badass: All the other characters get more and more paranoid and scared, while MacReady just gets more determined to kill the creature. He never freaks out and screams when he sees it, instead spouting one-liners and getting more and more determined to kill it.
He figures out how the Thing operates quite quickly and begins making very rational decisions to help stop its spread and identify it. Everyone else is quick on the uptake of the concept and generally avoid being Genre Blind.
He also Defies the Not Quite Dead trope and insists that even corpses be tested for being the Thing.
His first reaction upon hearing that something "Weird and pissed off" is in the dog Kennels? Call for the guy with the flamethrower.
What Happened to the Mouse?: The last time we see him, he's walking down some stairs. The original script had him getting attacked by a jack-in-the-box like alien, only they cut the scene as the special effects didn't look real enough, and Carpenter liked leaving it ambiguous anyway.
Pretty Boy: Somewhat younger and more effeminate than the rest.
The Stoner: Openly smokes marijuana joints in front of the whole group, spouts off on how aliens are real and "taught the Incas everything they know." His generally non-threatening and comical personality might be why the alien assimilated him, because he didn't draw much suspicion.
Moe Greene Special: He shoots the rifle-toting Norwegian through his eye.
Suddenly Shouting: Garry has just been proved to be human and not a Thing, and he is very annoyed.
Garry: I know you gentlemen have been through a lot, but when you find the time, I'd rather not spend the rest of this winter tied to this fucking couch!
Genre Savvy: Proposes to MacReady that everyone should prepare their own meals and eat from cans to avoid being assimilated.
Heroic Sacrifice: Implied. He is found as a charred corpse. Nauls immediately suspects the Thing did it purposefully, but others suspect Blair did it himself to avoid being assimilated. Being separated from the group long enough for the others to consider killing him on sight makes it murky.
Oh Crap: When he realizes that he dropped the keys that let the Thing get to the blood.
The Thing
The Antagonist, who tries to kill the heroes and make copies of them.
Alien Blood: Every cell of The Thing will try to defend itself, as opposed to regular human blood, which is inert. This is part of the basic nature of the Thing; even when it's replaced multiple people, each individual will act independently. In essence, every cell of the Thing is the Thing as a whole. In the original short story, people repeatedly make the mistake of assuming that you can't be a Thing if you attack a Thing.
Asteroids Monster: Every cell of the Thing is an independent organism. At various times during the film, it gets parts chopped off it, which grow new appendages and scuttle off.
Dangerously Genre Savvy: It knows it wouldn't stand a chance against a united camp (especially after nearly being killed already), so it sets out to spread paranoia, covertly destroy any equipment and cause sleep-deprivation until enough of its enemies are out of the picture.
Animalistic Abomination: It also spends most of the time disguised as a dog, and in the 2011 prequel it first surfaces as an insect/arachnid like thingie that may or may not be a distorted form of the alien pilot. In Campbell's novel, it is also implied it assimilated an albatross, and is now flying towards us.
Face Stealer: The Thing steals people's identities by absorbing them.
From a Single Cell: In theory, all it takes is one Thing cell to infect someone. Alan Dean Foster, in the novelization, seemed to think this is implausible, and has Blair talk in detail on the subject. Of course, that depends on whether you think Blair was still trustworthy at that point. Ultimately it's left up to fans to decide what pseudoscience to believe.
Harmless Freezing: The original Thing was frozen for around 100,000 years. When it's thawed, out it's completely fine. Childs even points out how impossible this sounds, to which Mac points out that it's from outer space and different from us. After the Thing abandons its plan to escape, its new goal is to kill all the survivors and simply freeze itself again until new hosts wake it up.
Hiss Before Fleeing: The Thing hisses and moans when it's currently in an imperfect copy of its host. The blood also finds a way to scream when it jumps from the petri dish.
Kill It with Fire: Fire is the only thing the characters have on hand that can kill the thing. Since it's a shapeshifter, shooting it would barely inconvenience it and it can survive for thousands of years frozen. Luckily, ice stations have handy flamethrowers.
Shock and Awe: Both Who Goes There and the unmade sequel miniseries have using electricity as a promenient way to deal with The Thing. One of the Boss-Things in the videogame was killed through electricity.
Losing Your Head: Decapitating the monster doesn't work, in one instance the head grows legs and walks away.
Manipulative Bastard: The Thing knows that paranoia only makes the situation worse for the humans and does what it can to spread that paranoia even more than it already does by nature alone.
Nigh Invulnerability: To destroy the Thing completely, not one single cell can be left alive, since even one cell is an independent organism with the power to assimilate an entire host body. The group burns them to ash, but realistically this would be unlikely to kill every single cell - which is why they haul the remains outside where they can freeze. During the scene where Blair is examining the dog-Thing, A deleted line had him mentioning that it was still alive. It's implied that this creature is the one that eventually got Blair. Unless he was infected during the autopsy.
No Biochemical Barriers: The Thing can infect Earth life just as easily as it did alien life. It's not clear how long this took or what the alien was like. Actually lampshaded in the original novel, where it's potential to infect us (or carry some alien disease) was initially dismissed based on the otherwise logical assumption that all of Earth's life forms, including plants and fungi, are more closely related to us than the Thing is, and their diseases can't affect us.
Non-Malicious Monster: It doesn't do all these stuff for malice. It's just trying to survive and remain alive.
One-Winged Angel: Whenever the Thing is exposed, it assumes very dangerous, monstrous forms to attack the protagonists with. Particularely noteworthy when it confronts MacReady at the end, and the Thing turns into a huge monster shapeshifter mash-up. Also an example of Clipped Wing Angel.
Paranoia Fuel: In-Universe. The Thing is all about creating this in the camp and turning everyone against each other, thus making it even easier to infect them all.
Starfish Aliens: There really is not a word other than "The Thing" to call it, because no one even really knows what it is. It is capable of perfectly replicating anything it has ever come in contact with, and every single cell of its body is a separate, hostile organism. It's so utterly alien that people aren't even sure if it has a true form or not, even the huge, grotesque monstrosity it forms in the end.
Tomato in the Mirror: Discussed. The survivors wonder, if the Thing perfectly mimics who it copies, does it even know it's a fake? The novel claims it does, absorbing the memories and personality of the thing, and Carpenter in the commentary agrees that if it did, it wouldn't matter - it'd use their personality to react accordingly. Given that the Thing-imitations take several actions to frame unassimilated humans, and one of them is secretly building a hovercraft, it seems probable that they know what they are. That said, the actor playing Norris mentions in the commentary that he played his character as being worried that he might be the Thing without knowing it. ( He's very much correct.) Note his reaction when offered Gerry's gun - "I'm not up to it.".
The Virus: Possibly the ultimate example. It can consume and imitate any life. Someone infected, assimilated and replicated by the Thing is such a perfect imitation that they never break character until either an opportunity arises for it to kill a bunch of people or it gets exposed. Even if other people get exposed as a Thing, a still-in-disguise Thing will remain in-character and even attack the other of its own kind, just to keep up the act. Worse yet, the monster apparently retains the knowledge of everything it's eaten (one isolated one is trying to build a spaceship to escape) and can even mix-and-match parts from the various creatures it's assimilated.
You Are Who You Eat: The Thing absorbs other people so it can assume their identity, leading to paranoia as to who is a thing or not. The alien can either consume a person in one go, but its individual cells also have the ability to slowly do this to an infectee, literally eating them from the inside out.
You Cannot Grasp the True Form: Nobody knows what the original Thing is or even what it looks like. Even John Carpenter notes that one could go crazy even thinking about it.
