"...you'd just attack me right now, so some of you are still human. This thing doesn't want to show itself, it wants to hide inside an imitation. It'll fight if it has to, but it's vulnerable out in the open. If it takes us over, then it has no more enemies, nobody left to kill it. And then it's won."
- Kurt Russell as R.J. MacReady
The classic John Carpenter science fiction film about an alien who kills and impersonates anyone, leading an Antarctic crew to heights of paranoia.