You play a gruff badass called Alex Mason, who early on gets captured and imprisoned in a Russian gulag alongside a man called Viktor Reznov, but (SPOILER!) it turns out Reznov is only a figment of Mason's imagination. The real guy died years earlier. See, you thought you were just shooting a bunch of dudes in the head, but it was your mind that just got blown (sorry).
Sure, astute players might have noticed that, aside from Mason, not one single person in the entire game talks to Reznov. Mason and Reznov are accompanied by several other soldiers, but none of them so much as even look at Reznov, let alone question why a freaking Russian is hanging around guys tasked with exterminating as many of his countrymen as possible, one shotgun shell at a time.
They could've at least asked him where he got that sweet-ass jacket.