THE DAY AFTER TOMORROW was on HBO tonight, and reminded me of Chekhov's assertion (I'm paraphrasing here) that if in the first act of a play you see a gun, by curtain it should go off.
This device can be effective and a lot of fun when laid in well, but too often leads to silliness like DAY's wolves, introduced in scenes completely outside the main thrust of the story (Zoo Guy: "The wolves! They're gone!") just so they can show up later to menace Jake Gyllenhaal.
The movie has another unfired gun: Emmy Rossum's injury. We see her get cut, and she winces in pain in a later scene, and that's it until the wound becomes serious enough that Jake has to go out and face -- wait for it -- the wolves.
Though more a part of the story's spine than the zoo scenes, I think the injury device jars because it's treated implausibly. Emmy's character is established as a smart girl, so why wouldn't she realize she was sick, or where that sickness came from? And she's not set up as being overly stoic or a martyr, so why didn't she mention the wound before it went septic?
Things That Become Important Later can give a story continuity, depth, and surprise, or they can become a distracting game where the audience identifies the item early on and just sits and waits for it to pay off.
Thoughts? Any favorite examples of unfired guns that work, or don't?