Life is poorly written. The objectives are vague, the plotting is sloppy, and characters enter and exit without any logic or any guarantee we’ll see them again. Even if you hold there is some kind of god-like author up there, even the most devout will point out that he moves in mysterious ways. In life, good men die, liars prosper, or sometimes the opposite or any other permutation, and we’re all just one aneurysm away from an arbitrary and meaningless death.
Movies, by contrast, are models of a moral universe, on constructed by a writer, a logical screenwriting god intent on making a point. By understanding this, you understand theme, the ways movies differ from life, and how you can use theme to build your script in a logical and efficient way. The moral of a movie informs every atom of its construction.
Writing should have a sense of unity to it. Unity is a lofty word that basically means “don’t be arbitrary.” You know that old meme that goes “I see what you did there?” That’s writing in a nutshell. We always want to see what you did there. Every element in a script should have a purpose and intelligence behind it, even if we can only see it after all is said and done.
Themes allow stories to have a sense of unity. All movies are propaganda for a moral. That said, the moral might not be reassuring. On a meta level, some movies actively work to eschew any sense of meaning. Even in these examples, they can be judged by how arbitrary they seem to be. If a movie aspires to seem random and pointless and it succeeds its done a good job. If some kind of moral ends up bleeding through, it has failed. If you hold that movies must illustrate a theme, all elements in movies must affect the values of the theme in a positive or negative way (even the evil villains in a movie about heroism help illustrate the theme). When you chose your theme, you are making a cogent, powerful, seductive argument for the world being the way you see it. To this end, every line of dialogue, character choice, action, and image should work to sell the overall point you’re trying to make.
Unlike life, stories are driven by plot and built on a logic. Real-life humans can remain static their entire lives, story characters are challenged and changed with every passing sequence. Movies are well organized and carefully plotted, life is not. Movies have an endpoint, life perpetuates itself like a a bad soap opera. Movies end on a thematically appropriate image, the universe will outlast mankind, our sun, and endure through an eternity of timeless entropy. Thank goodness for movies, it’s nice to live in a world where things make sense, even if it’s just for a hundred pages or so.