In the movie, senior biologist Blair (A. Wilford Brimley) and physician Copper (Richard Dysart) address the mechanics of the Thing, albeit in a limited way. “What we’re talkin’ about here is an organism that imitates other lifeforms,” observes Blair, “and it imitates ’em perfectly.” It attacks, absorbs, and shapes itself as a carbon copy of its host, indistinguishable from the person it targets. You don’t need a whole taxonomy classification to understand that; as Richard Masur’s Clark says, “It’s weird and pissed off, whatever it is.”

The Thing can’t be detected unless it’s compelled to reveal itself (such as in the celebrated blood test scene), and several cast members questioned if and when their characters knew they were infected. David Clennon, who plays assistant mechanic and resident stoner, Palmer, recalled hours of rehearsal time spent “discussing f***ing metaphysics” about the infection:

“Some of the actors were obsessed with this question: When you become the Thing — when the alien takes over your mind and body — do you know that you’ve become the Thing? Or do you just go on thinking that you are your old self? I couldn’t see the point of solving that silly riddle.”

Carpenter remembered Russell as being the most persistent with this question. He told LA Weekly:

“The big question that kept coming to me was: If you were a Thing, would you know? I think Kurt Russell started that one. I said, ‘I think you would.’ But he kept asking that question, so I don’t think that answer was sufficient.”

It’s an answer /Film’s Eric Vespe keeps in mind as he explores the theory that MacReady was not only infected at some point, but he knew it. Dive down that rabbit hole here.