We hear the same basic principle about screenwriting all the time -- the protagonist needs to want something and it needs to be hard for him or her to get. If you're not sick to death of this axiom, you probably haven't been writing for long. (And yet it's never a bad thing to hear, because it really is true and a script that ignores it can go off the rails very quickly.)
But that's not the only way to look at a script. That's just the narrative perspective: creating a difficult quest for a character to succeed or fail at. Simultaneously, we're doing something else. We're challenging an idea. It might be a fairly simple idea ("Good triumphs over evil," "The truth will out," "Crime doesn't pay"), but it needs to permeate the story and it needs to be tested every bit as rigorously as the protagonist is. That means stacking the deck against it whenever possible, just like we do with our characters. At every turn, we want it to be easier to embrace the opposite of our idea rather than the idea itself.
An example. Let's see. How about Die Hard? (I know, I always go to that one. One of these days I'll see another movie, I promise.) Like many action movies, this one deals primarily in the principles of "Good triumphs over evil," and "Crime doesn't pay." And it does a great job of testing them. Look at those villains: a crack team of expert thieves with an arsenal of firepower and demolition equipment at their disposal. They're smart, they're ruthless and they've done their homework. Then there's the FBI unit sent to take them down: a bunch of overconfident, shoot-first lunkheads who seem bound and determined to blow the mission at every turn. Halfway through the movie, we have to be thinking... this is going to be the story that teaches us that good heroes triumph over evil criminals? And that's perfect. Like the protagonist, the principle at the heart of the film must always be in jeopardy. Both audience and protagonist must always be tempted to accept the opposite of that idea.
But then -- right in the end, when it counts the most -- the central idea proves its worth, exposing the fatal weaknesses in any arguments against it. And it is stronger for all the challenges it has faced to its validity -- even if we could have accepted it at face value from the beginning.