|Picture by Petar Milošević|
Filmmakers will often sing the praises of what's called "the ticking clock". In a nutshell, it's the concept that there's a countdown or a time limit. Do X in this amount of time or else Y will happen. The idea is that the ticking clock will force urgency and action, and at the same time create tension in a scene. Used well, it can work wonders, so much so that it's a staple of the thriller genre.
But a ticking clock is not always a good thing. As soon as you set it, you tell the audience how much time is left before the end of the book-- and often we're looking at the clock and deciding whether or not epicness can actually happen in that amount of time.
I'll try to avoid spoilers, but if you've been to the movies recently, you might have seen two titles in particular: Brave and Men In Black 3. Both of these feature a literal ticking clock, to both of their detriment.
One essential scene in Men In Black featured a time limit of two minutes. It was supposed to make things exciting, but suddenly all tension drained from the moment. It's really hard for a lot of things to be condensed to two minutes and be particularly epic, and the scene in question was rather boring because of it.
Brave suffered a similar problem: without going into too much detail, we were given a time limit of two days. Instantly my face fell. Pixar works are renowned for their incredible character development, but the kind of character development and change we needed in this picture wasn't the kind that could be accomplished within 48 hours, even by the most talented of writers.
Don't get me wrong-- a ticking clock can be used well, and when it is, it adds tension like none other. I've mentioned before how much I love Mission Impossible: Ghost Protocol, which made excellent use of the device. The Lord of the Rings includes another, especially in The Return of the King, when Frodo and Sam are unwittingly racing against time to save and be saved by Aragorn's forces.
In MI:GP, there wasn't one ticking clock but several in a row, which started out relatively small and got larger throughout until they reached epic proportions. Because the entire plot didn't ride on each countdown, we actually believed failure was an option (something we didn't get from either Brave or MIB3), and that left our hearts racing.
In tRotK, our heroes didn't actually know there was a clock to race against. The fact that we the readers/viewers saw the big picture meant that we felt an added degree of tension, and a whole lot of us were yelling at the pages of the book to hurry and do what they came to do, before our other beloved characters were lost.
These aren't the only ways to make it work, of course, but they're original and creative ways of taking an old trope and using it to pump adrenaline into seasoned readers.