This post gives away major "big reveals" for the popular works it references, but most of them have been around for so long that, if you haven't heard of them yet, you were probably never going to. So really I'm doing you a favor. You're welcome.
Scabbers the rat (yes I'm a grown man referencing Harry Potter) is just a tertiary comic relief that squeaks, nips fingers and knocks things off shelves whenever J.K. Rowling sees fit to deviate from more compelling action, and you quickly forget about whatever minor appearance he makes after reading on for five pages. He's been a treasured pet in Ron Weasley's family for twelve years, because they're poor. It's cute, we get it. Moving on... but wait, as it turns out in Book 3, the real traitor who sold out Harry's parents to their deaths got away with it by going into hiding for twelve years in the form of a tiny, furry, what! Are you shi**in me!?
Kyle Reese travels back in time to save Sarah Connor from the Terminator because she's supposed to live on, find a nice man, and give birth to mankind's savior before a nuclear holocaust leaves the earth fodder for menacing machines. Soon, their riveting high-speed escape from the shotgun-toting, motorcycle-straddling Schwarzenegger fades behind their mutual vulnerability, and a passionate love story takes center stage in a way so authentic that it lacks the contrived feel of throwing you a pen to connect the dots. And when you finally see a pregnant Sarah driving off into the unknown before the credits roll, you know Reese got the job done in every sense.
When Hannibal Lecter cunningly bludgeons and slices his way through his cage and its two inept guards, still in a building crawling with cops, you know there's no way he's getting any farther. The boys in blue charge into his quarters with guns drawn, finding one of their own hanging disemboweled on display and the other sprawled on the ground, face shredded, barely breathing. The camera stays on him as he's whisked away, convulsing on a stretcher and crammed into an ambulance that hauls past every road block and security checkpoint, until, after the cops find a corpse in Lecter's clothes, missing its face, the hospital-bound guard sits up in perfect health, removes his mask of loose bloody flesh to reveal himself as none other than...? You mean... he was right under everyone's nose the whole time, wearing a dead guy's face? That's so Lecter.
Kevin Spacey's portrayal of Verbal Kint, the meek, ineffectual "gimp" member of The Usual Suspects serves to describe to the cops, and to us, the ever-present, ever-threatening devil in the shadows Keyser Soze who purportedly slaughtered everyone who ever wronged him or interfered with his business. His story is so convincing that they release him, and, well, by the trend of this post I'm sure you get the common thread I'm highlighting, but this shocking revelation was so well-done that subsequent stories (like Fight Club) whose final twists hinge upon characters being someone else entirely, were noted as pulling "the Keyser Soze move."
Darth Vader... enough said.
And to quote The Lonely Island, "When Bruce Willis was dead, at the end of 'Sixth Sense'..."
I've always considered it the mark of a master to dangle something so powerful and essential to a story right before the audience's eyes, keeping them on the hook with enough bait but never letting them piece together the secret being exposed inches in front of them until finally, at just the right moment you hit them with it so seamlessly it's like your hand was never even there, and they reel back amazed that they didn't catch it sooner.
If executed properly, it can unify what seem like arbitrarily scattered fragments into a bright mosaic that slaps the confounded viewer with a gratifying kiss not soon to be topped.
It's a jarring impact that will make them share your work with their friends just to re-live the wide-eyed, sweaty "No way!" that it brings.
So, is this a technique I've woven throughout my own book, learning from the best to conceal a world-shattering secret in plain sight of both my protagonist and audience until the most climactic delivery its buildup could support?
God damn, what a let down this whole post would be otherwise, like "Contact" where you're following along with Jodie Foster the whole time just to see the aliens, and it turns out to be her father's ghost or something... I didn't actually see it.