Both film's safe zones are not an option. Meaning, the characters of The Thing couldn't just take off, they would die in the frozen desert spanning hundreds if not thousands of miles. The characters of Alien were surrounded by the perils of space with only a small shuttle incapable of carrying the entire crew as an option.
Granted Alien takes place several million more miles than The Thing's remote location, but both film's locale is simply too far for any quick rescue or a convenient calvary racing in to save the day.
Both films have no easy escape route. They're both stuck in the confines of a small area and forced to deal with their respective creatures.
Both aliens were seeded into the main characters' closed-in confines from their respective alien ships. More accurately for The Thing, it was discovered within close proximity of it's ship. Of course it was the Space Jockey species that appeared to be in control of their ship, and the xeno eggs were just passengers so to speak.
The same for The Thing, popular speculation suggests that the ship frozen in ice might have been another alien species that was attacked by the Thing alien. On that note, that's the very visual cue we see in Alien regarding the Space Jockey with the hole in it's chest implying it too was attacked by a 'passenger' in it's own ship.
Both aliens were thousands if not millions of years old before being discovered. They were lying dormant for eons until some curious humans came along and figuratively poked a stick at them.
In both cases, the creatures were hand delivered by the unsuspecting humans into their cramped corners where the showdowns begin.
No cuddly ET's here. Both creatures mutilate and slaughter their victims with a savage assault. While Alien is less graphic, both creatures make a bloody mess of their victims.
Both aliens were not depicted as 'evil,' simply doing what they need to survive. They are neither power hungry nor do they desire to 'rule the people of Earth.' Like any living creature they do what it takes to live, including killing any humans looking to flesh them out and exterminate the creatures.
Sure, this is nothing more than simply a visual similarity, but it was these two films that set the tone of future films to showcase close ups of fangs streaming with a generous supply of drool. Gone are the days when menacing looking dry canines were considered terrifying. If a film maker today doesn't equip his star creature with a saliva gushing machine, his film is doomed to fail at the box office.
Both aliens at one point take on the visual likeness of spiders. Alien's facehugger is like a spider with a tail and The Thing's spider was an aberration from hell - a victim's head, antenna eyes, and spider legs, which earned the nickname by fans known as Spider Head.
Both aliens physically transformed into other shapes quickly. For Alien, it was a matter of hours changing from facehugger to chestburster to full size xenomorph. In the Thing, physical transformation happen much quicker, in a matter of moments as we witnessed on screen.
Just like the Jack in the Box toy, both aliens are shown popping out of a victim's chest with buckets of blood spraying every which way. The Alien chestburster poked it's head up during the crew's dinner and The Thing's alien burst out of Vance's chest when Copper was trying to revive him from an apparent heart attack.
Both aliens use stealth in it's battle to overcome it's human adversaries. In Alien, the creature is using the ventilation ducts to move around and biding it's time to attack. The Thing's alien combines stealth with subterfuge and even sabotage. A number of it's acts of stealth damage are not even seen on screen such as the destruction of radios and other electronic/mechanical hardware.
In both films, the agenda of the two aliens is to attack their victims once they are seperated from the rest of the characters. And in both films, the two aliens eventually reveal themselves to more than one character of their respective films.
In Alien, Brett was alone in the Undercarriage Room when the creature attacked (if you don't include Jonesy the Cat). In the Director's Cut, the creature immediately takes off into the air ducts with Brett's body when Ripley and Parker appear in the room. The creature eventually reveals itself to more than one character of the film when it approached Lambert while Parker was also present.
The Thing alien also bides it time waiting for a victim to be alone. An example would be when the dog/alien wanders into a victim's room while he is alone, we only see the silhouette of the victim. The attack is never seen, but it's heavily implied with the set up of that scene. The creature eventually reveals itself to more than one character of the film when it burst from Norris' chest while a number of the characters were also present.
Both aliens take on the characteristics of their host/victim. Okay, maybe this is not an 'identical' match, but the general imitation theme is what makes them similar. While we never really see this in Alien 79, other Alien films and novelization give examples that the Xenomorph adopts some of the characteristics of it's victim.
Examples include Alien 3, where it has more of an animal's behavior and posture since it's victim was a dog (or Ox for the Assembly Cut), and the much hated AVPR spinoff where the 'Predalien' xenomorph adopts the mandibles and dread locks of the Predator species. It's The Thing's alien that truly imitates it's victim much more accurately. Providing you don't catch it in the act of transition. Then you might witness a human with some freakishly long and deformed fingers.
Both aliens showed a priority in propagating it's species (well, at least for the Alien director's cut). The Alien 79 creature does more than just attack it's victim, as we saw in the director's cut, it uses it's victim to mutate to a Xeno larvae for a lack of a better description. In the Thing, Blair gives a description of the Thing alien's ability to propagate globally in a relatively short time. Meanwhile, the creature is a busy bee bent on consuming and duplicating everyone at the American station.
Both aliens can multiply without insemination or coupling. No romance needed for either of these creatures, they get busy all by themselves in their quest to produce a gaggle of more facehuggers and little thingys.
Both aliens use the victims as hosts. In the Alien Director's Cut, we see Brett and Dallas in different stages of host mutation, often called cocooning. The Thing alien also requires a transition from host to fully duplicating the victim.
Both film's main protagonists were initially not in charge, but were later forced into bringing forward the most leadership. Once the commander of the ship, Dallas, met his demise, Ripley took the initiative to investigate what Ash was hiding.
She also guided the remaining crew to continue with Dallas' plan to flesh out the creature and later orchestrates an escape to blow up the Nostromo and take their chances in the shuttle. Not to mention she dispatched the creature all on her own.
In The Thing, Garry is in charge of the American base in the Antartica, but it was helicopter pilot MacReady who took charge after his mates turned on him. He conducted a blood test on the others to isolate who is human and who is alien. MacReady also managed to inflict a heap of hurt on other characters that were 'consumed' and duplicated by the alien.
In both films, characters Dallas of Alien and Blair of The Thing consulted their computers only to receive bad news. Dallas, when seeking info from his computer, received no help in surviving the alien threat, the camera switches to Dallas with a look of despair. Blair, when seeking info from his computer, received grim news in surviving the alien threat, the camera switches to Blair with a look of despair.
Both film's protagonists had a non-creature nemesis, or more accurately - other characters they regarded with distinct distrust. Ash for Weaver, Childs and others for MacReady. In Alien, Ripley was showing signs of distrust for Ash early in the film considering his actions were not exactly 'by the book' for a science officer. When Ripley discovered 'Special Order 937' - Ash's instructions from the corporation to preserve the alien specimen even at the cost of the crew's lives, it was clear to her she was now faced with two dangerous beings as a threat to her life and the remaining crew.
For The Thing, Childs and the others allowed their paranoia get the best of them and were out for MacReady blood. Childs was the most aggressive of the bunch testing Mac a number of times, so MacReady too like Ripley, had to contend with both the hostile alien and threatening non-creature/humans.
Both films climaxed with a fiery explosion in an effort to destroy their respective creatures. True enough the Nostromo explosion was on a much grander scale, but the theme is still there, using explosive force to rid themselves of the persistent aliens. In Alien, all that WMD fire power was a failed effort since we found out later the creature was tucked away in the shuttle. The Thing's alien and it's demise is left unknown, but at the very least it was at the center of MacReady's showcase of incineration.
Both films situated the victims with limited and lackluster weapons. All I know is if I ever have to face hostile aliens, no flame throwers that crap out at the worst time, lame cattle prods, or hand guns for me. I'm going in with the BFG weapon (Big Fucking Gun) seen in Dwayne Johnson's video game movie adaption and box office dud, Doom 2005.
Sci-fi Horror Greats
While some of these similarities could likewise be applied to other films, it's the quantity of common themes between both films that stand out. Also, these visual and storyline concepts are not exactly original. Dan O'Bannon stated in an interview that he borrowed from a number of fifties era sci-fi films in coming up with the concept of Alien. For The Thing, it took it's pieces from the Campbell novel, not to mention certain shared scenes with Hawk's 1951 film version.
In any case, both films have been put on a pedestal for many years as landmark classics and any film fan would struggle to come up with a list of films that are on par with these two when it comes to captivating sci-fi horror.