Screenwriters hate following rules. Doesn't matter what stage you're at, really. If you're new at the trade, the "rules" you read in books and hear about in classes seem bizarre and draconian, a total wet blanket on your creativity and zeal. If you've been writing for a while, you might think that the rules apply to people writing a certain kind of script, just not to you since yours is different. And if you're a veteran writer with several produced screenplays under your belt, you're sick of following the rules and are ready to tackle something outside the box.
I'm still learning the rules, as many of my dispatches here imply, and trying to understand what they mean to me and how I can apply them. The concept of the character arc has been one of the most difficult for me to grasp. I've understood for a long time that it's important for characters to change in meaningful ways over the course of a screenplay, I know that it's a staple of most successful, award-winning scripts, but the specific "why" of all that never jelled for me -- until yesterday.
So let me take a stab at explaining it in my own words.
What's the best thing someone can tell you when recommending a movie? Okay, maybe "four stars," "best picture of the decade," or other such superlatives come to mind. But I wouldn't say they're the best things that can be said about a film. Instead, I'd argue that the best thing you can say about a movie is this: "It changed my life." If someone tells you this, especially someone you trust, you're going to be hard-pressed to avoid seeing the film in question. You might even be hesitant to see it because you're apprehensive about what effect it will have on you. In any case, it's probably one hell of a movie.
Now let's inspect the statement a little more closely. If someone says a movie "changed [her] life," then something about her life and the way she lives it is different now than it was prior to seeing the movie. This, then, is her "arc." Her life was at point A; now it's at point B; the film brought her there -- it changed her. And that change proves the movie's power.
So, okay, we'd all like for our work to be this powerful. Unfortunately, as writers we have very little say over the emotional reaction of the average reader or audience member. That person is bringing his own baggage, his own perspective, to the experience; and those intangible elements -- neither of which the writer has any control over -- will hugely affect how he responds to the movie. That's the bad news.
The good news is that we have absolute control over every one of the characters in our script. Therefore, we can choose whether or not their lives are changed by the story. And if we make it a life-changing story for all of them, then we've got a much better chance of impressing our readers and audience. They know the best stories are the ones that change them, and now they've seen multiple lives change before their eyes. We've told them a good story and given them concrete proof of its effectiveness. That's one reason why character arcs are important.
(A final note here. Conventional wisdom on arcs is that the villain/antagonist shouldn't have one -- he is undone by his own inability or unwillingness to change, and his lack of arc helps to put the hero's arc into sharper focus. That's one way to go, but definitely not the only way. One example that comes to mind is Serenity, in which the villain not only suffers a physical defeat but also chooses to abandon his rigid philosophy as a result of what he sees. In the DVD commentary track, writer/director Joss Whedon explains that he thinks having the good guy show the bad guy he's wrong is more effective than just killing him. This approach may not work for all stories, but it's effective in this case and, I think, worth considering.)