For me, the scariest moment in 1956's Invasion of the Body Snatchers comes relatively early in the film. The protagonist, Dr. Bennell, brings his friend Jack and a psychiatrist, Dr. Kauffman, to his girlfriend Becky's cellar where the night before he'd seen one of the pods slowly turning into her duplicate. Bennell seeks to prove to Kauffman the town is being replaced by the sinister duplicates. Jack doesn't need convincing; he'd seen his own duplicate the night before. Both Dr. Bennell and Jack hastily affirm seeing the duplicate Becky in Kauffman's presence . . . only to see, on closer inspection, it's only a wad of blankets. Kauffman is then able to use Bennell's and Jack's quite real mistake to show them how they're entirely deluded. Considering how insane the idea is of the town being replaced by pod people, it seems plausible that this works and Bennell and Jack thereby take part in pulling the wool back over their own eyes. The takeover is quiet and its use of human psychology is insightful and subtle.
But why fight it? Bennell (Kevin McCarthy) and Becky (Dana Wynter) are both divorcees; they're in a good position to know that pod Kauffman (Larry Gates) is right when he says human emotions are messy and impractical. Just as it's easy for Bennell and Jack to believe the pod Becky wasn't really there, letting the invasion proceed is as easy as falling asleep. Eventually it becomes painful for Bennell and Becky to stay conscious.
Aside from what's actually happening to the townspeople, there's also the horror of isolation. Bennell has to watch as everyone around him is swiftly and complacently changing, and everything he held as normal and rational the day before is suddenly irrational in this new world, to the point where he ends the film raving like a madman in the street. In the framing story added to the film because of a studio afraid of an unhappy ending, Bennell even ends up being treated as crazy when he's away from the pod people. He internalises the impression other people, the pod people, have of him, against his will. Can he help it? These were the faces of people he trusted all his life.
Once the aliens have taken over, will it really make much difference without a Bennell there to see it? As invasions go, this one is remarkably lacking in violence. One group of people is replaced by a more serene but otherwise identical group. Bennell fights only in the interest of self-preservation; the pod people see their takeover as objectively rational.
I love how shots of the alien pods are often subdued. There's no close-up on them here when Bennell finds a couple have been stashed in his car. People carry them around like any other item of produce; it's just something else delivered for everyone to consume without thought.