I'm a pretty big fan of Mission: Impossible 3, though I acknowledge that my opinion is colored by my overall enthusiasm for the work of J.J. Abrams in general, and the TV show Alias in particular.* Some people have criticized MI3 by saying that it's just a big-screen version of Alias, but to me that's what makes it great, and Tom Cruise clearly thought that the sensibilities of the two franchises were compatible when he hand-picked Abrams for the gig.
I bring this movie up because it's one of the few big-budget action thrillers in the last several years that has actually taught me a useful lesson about screenwriting. It's not a very complicated lesson, but it's a useful one and it applies to pretty much every genre. Since I can't find the script online, I'll describe the scene in question here:
Outside a remote German factory building, Ethan Hunt and his team have just staged a complex assault on a villain's hideout in order to rescue a kidnapped CIA trainee, Lindsey Farris. Ethan gets Lindsey aboard the helicopter and the team takes off, only to be pursued by two more helicopters piloted by some of the villain's men.
Pretty good so far, right? This was all quite thrilling on the big screen; I don't know how it looked on paper. I do know this, though: it's pretty standard action-flick stuff up to this point. Let's keep going:
Ethan's ace pilot maneuvers the helicopter through the spinning blades of an enormous windmill farm to escape the bad guys. One of the bad-guy helicopters crashes; the other manages to keep up.
Then -- all of a sudden -- Lindsey grabs her head in pain, screaming that she hears a loud noise (no one else can hear it). Ethan performs an x-ray scan with a handheld device and finds that a tiny bomb has been injected into her brain -- and it's been activated -- and she'll die if he can't defuse it in time.
Now things have really gotten interesting. On top of the helicopter chase, there's the imminent danger of Lindsey's brain-bomb. The chase is big-scale and threatens the entire team; the bomb is small-scale and threatens only Lindsey. Both elements are thrilling on their own, but the fact that they're happening at the same time, and in the same space, elevates the tension to an almost unbearable level.
Those bolded words up there are the crux of this whole thing. Most of screenwriting is, to use an industry buzzphrase, execution dependent -- meaning that just writing something (a script, a scene, a line of dialogue) is meaningless; you have to write it well to get any credit.** However, the example I've cited is an exception because the mere existence of those two elements side-by-side makes the scene thrilling. As a writer, you'd win points for that scene even if the execution were just so-so. To me, that makes it a pretty great technique to know about.
But there's more to this trick than just adding stuff to other stuff, and that's why I'm calling it layering. You need the elements of the scene to fit neatly together. In this case, the writers started with the helicopter chase and then inserted the brain-bomb crisis. The two elements are wildly different in scope (helicopter: huge, threatens entire team; bomb: tiny, threatens only Lindsey), which makes it possible for them to co-exist onscreen without being totally overwhelming or numbing to the audience. (Simply adding more pursuing helicopters to the mix would elevate the danger, but after the initial thrill of "ooh, there's more helicopters chasing them," the audience isn't going to perceive the threat much differently.) Furthermore, the brain-bomb has the added benefit of pulling on the audience's heartstrings in a way that the helicopter chase doesn't. Putting a single character in peril, rather than a bunch of them, is almost always more emotionally powerful.
The key to successful deployment of this technique is being aware of how often you can get away with using it. Too many scenes like the one above would exhaust the audience to the point of numbness; they'd cease to be surprised or thrilled by anything else in the movie. In MI3, there are only a few instances of layering; the majority of the action scenes focus on one element at a time, and as such they just have to rely on good old-fashioned execution.
*Really just the first two seasons of that show, to be even more particular.
**Actually, "execution dependent" usually refers to the kind of script that may be well-written but would require skillful acting and filmmaking to turn into a successful movie. Most studios and producers tend to reject that kind of material in favor of something that would be a hit even if it were directed by a blind guy and acted by the cast of a a local morning news show.