If a gun is introduced in the first act, it must be fired by the third.
The theory of "Chekhov's gun" comes from a Russian playright named Anton Chekhov. It's a method of foreshadowing what is to come -- if something that seems irrelevant pops up in your story, you can be fairly certain it will become relevant by the end. (Or it won't. In which case the author/screenwriter/playright was just dicking with you.)
The most obvious example of Chekhov's gun that comes to mind is Breaking Dawn by Stephenie Meyer. I was a summer associate in Fake Austin in July 2008, and I kept hearing about a book that was being released. I was vaguely aware of the Twilight phenomenon, thanks to my sister Echo, but I had never before experienced the kind of hype and mania that accompanied the build up to Breaking Dawn. It was like the time my friends and I got tickets to a midnight showing of X-Men 2 freshman year in college to celebrate being done with finals and we were surrounded by nerds who gave standing ovations throughout the movie--only worse.
The night of its release-party, my firm was holding an event at a Dave and Busters, and after an evening of buffalo wings and Dance Dance Revolution, I decided to pop over to the next-door Barnes and Noble to see what all the pre-midnight fuss was about.
It got weird.
It got real weird.
Needless to say, I left that Barnes and Noble without a copy.
But then the next day I was at the gym and I noticed (I kid you not) a 6 foot 6, completely ripped guy my age ... reading Breaking Dawn as he worked the stair-stepper machine. I watched him as he tore up and down the stair-stepper, working up a sweat ... only to inexplicably slow down as he became more and more engrossed in the story.
Well, after that I drove straight to my nearest bookstore and bought a copy. You would have too, if you'd witnessed what I'd witnessed.
As I sat by the pool in my complex that afternoon, stubbornly enduring the 110 degree temperatures, I read the section where Edward tells Bella, apropos of nothing, about the prohibition on making children into vampires.
I knew in that second, roughly 20 pages into the book, that by the end of Breaking Dawn, someone was making or having a vampire baby. (And since Stephenie Meyer is Mormon, probably having. We do love us some procreation.)
No wonder that kid had been unable to focus on getting his shred on.
Another example of books that drop early details that become incredibly relevant later is the Harry Potter series. Anyone else remember a Harry Potter and thinking, "Of course he's a werewolf! Of course the rat is really a person! Of course the diary is evil! Of course he's not really a professor! Of course the wand belongs to someone else! Of course she knew Snape from before! Why didn't I see that?"
Because JK Rowling knows how to scatter her guns amid all the other world-building details that she provides to make the world of Hogwarts come alive. She lulls you into a false sense of wonder with tidbits on pumpkin pastries, floating candlesticks, and the history of witchcraft in the Bronze Age -- meanwhile, don't pay attention to this detail, it's just the remedy for an accidental poisoning.
OR IS IT?
Do you have any memories of a book, movie, or film where suddenly you knew what was coming -- or better yet, the delight you felt at the end of a book, movie, or film where you realized you should have known, but didn't?