Decoy Protagonist: While engineered by the plot to be the Final Girl, it's Marty who does all of the thinking and heroics during the climax of the movie while she mostly freaks out. She starts to pull it together toward the end, but by that point the world is literally minutes away from ending.
Badass: He's the only member of the group who succeeds in accomplishing anything, and actually manages to beat the organization in the end. He also racks up an impressive body count compared to every other character, who each only kill one monster at best and none at worst.
The Cuckoo Lander Was Right: Marty was surprisingly on the money about a lot of things even before they started to go to hell. Apparently, his pot has made him mostly immune to the controllers' attempts to control him
The Fool: Marty is called this by many, but he surprisingly fits into the Tarot archetype beyond just being a hippie stoner—he manages to succeed where others fail, often by pure luck.
Genre Savvy: The first to realize something isn't right.
Wrong Genre Savvy: Although, even after encountering the Buckner zombies, Marty finds a hidden camera and concludes, "I'm on a reality TV show!" It's a Justified Trope in that he's on pot... and the control room actually treats the situation like a reality show.
Get a Room!: Tells this word by word to Holden and Dana.
The Hero: More or less becomes this by the film's third act, throwing the movie Off the Rails and foiling the organization's plot.
Hidden Depths: He's shown to be astoundingly philosophical during the final scene. Before that he proves to be both great at thinking on his feet and a surprisingly adept fighter.
Only Sane Man: Marty, who keeps cautioning the group against actions like reading the mysterious Latin. His pot-smoking has made him Properly Paranoid as well as resistant to the mind-altering chemicals used by the villains- mainly due to a mistake on their part.
Death by Irony: After spending the entire film wishing he could see a merman, Hadley is finally done in when a merman enters the control room and chews his face off. Hadley is pretty aware of the irony, too; his last words are "Oh, come on!"
In Universe Nickname: "Aquaman," used around betting time as a reference for Hadley's desire to see a merman in action.
Covert Pervert / Not so Above It All: During Jules' sex scene, Truman can be seen making a few discreet but noticeable glances in the direction of the screens. He also is notably holding a clipboard in front of his crotch in that particular scene
Leaning on the Fourth Wall: They are pretty clearly a metaphor for the audience themselves, especially of horror movies: If their specific demands for the scenario aren't met the whole world ends, in the same way a work of fiction fails whenever it's audience isn't pleased with it.
Big Screwed-Up Family: Apart from the casual emotional abuse and sadomasochism metted out on a daily basis while they were alive, the Buckners eventually went from murdering travellers to mutilating and killing each other. In short order, Pa tortured Ma to death by cutting her belly open and filling it with hot coals; Matthew killed Judah; one of Patience' arms was cut off and fed to the rest of the family; Matthew killed Pa, specifically by mutilating his jaw; finally, he presumably killed Patience.
Cannibal Clan: A possibility. They certainly didn't have any qualms about hacking off Patience' arm and eating it.
Nice Job Fixing It, Villain: Patience Buckner eventually finds her way down the ritual chamber and kills The Director with a hatchet to the head, before falling to her doom herself. This didn't really fix anything since the world was ending anyway, but it did give Marty an extra few minutes of life.
Our Zombies Are Different: They are a "Redneck Zombie Torture Family", incomparable to regular zombies as an elephant is to an elephant seal. Mostly they are Revenant Zombies, being clearly retaining their intelligence and motivated by the desire to torture and murder rather than the desire to eat.
Nothing Is Scarier: In the film, we don't know what he does. Although according to the Cabin in the Woods wikia, the film reel that Marty brief looks at, while in the basement, was suppose to bring out Kevin.
Graceful in Their Element: In it's only scene, the Merman has great difficulty moving on dry land, and was only able to kill Hadley because he'd been knocked to the ground by an explosion. However, judging by the presence of a lake near the cabin, it presumably would have fared better had it been chosen for the ritual.
Ludicrous Gibs: "Dude, be thankful. Those things are terrifying. And the cleanup on them's a nightmare."
Our Mermaids Are Different: Sporting a huge fanged mouth, long tangled hair, webbed fingers, a fishtail, and a blowhole, the Merman seems only vaguely human at best.
Alien Blood: When one of them is shot in the head by Marty, it bleeds green gunk instead of blood.
Artifact of Doom/ Summoning Artifact: Apparently, a large chest of bottles and vials containing unknown substances, complete with an antique gas mask. At present, it's not known if the bottles had to be opened or the gas mask had to be worn (or both at once) in order to summon the mutants.