Stories are about one thing: a person's ability to change. By the end of the story, you should have a different character than you started with, or you should have watched someone fail to change in the face of events that affect them. You'll find that stories which lack this focus feel hollow and empty. You might come away wondering what the point was, even in the most seemingly exciting or epic of tales.
This is what can reduce a superhero film of Earth at risk down to a yawn. This can make a chess match become more riveting and intriguing than any heist scene. Similar to death and taxes, change is the only constant in our world. So, how do you make sure you capture this nebulous concept?
There's two techniques you can use for better displaying what is going on in your stories.
First is the “Do It for the Cool” rule, which comes down to questioning every decision you make along the way. When you’re including this awesome scene of a car smashing into the side of a moving train, is there a purpose? Or is this scene only in here because it's really cool? Does my character, in the midst of an argument with their lover, say that particular line for any reason other than it sounds really cool?
Cool moments and scenes are definitely the things we remember and stick out in our minds, but our minds also lie to us because they're intentionally forgetting two things: all the non-cool things that built up to the cool thing, and the consequence of that cool scene. The change that went into it and the change that comes out of it.
You should try to always consciously evaluate every decision you make to wonder why you are including a scene, then play out what happens next according to the character. If you have so-and-so, your dashing hero, knowing what you know about so-and-so, how do you think they would react next to that thing they just did? How would they react to that thing they just saw? That betrayal by their partner? What next? You might have just changed the whole course of the story because of it, but that's how it has to go, otherwise that scene is just there to look cool and didn't affect change.
The second technique is final character mapping. This is a modified mystery technique you could use, in which you know who did it and work backwards. In this instance, pin down who your character is by the end of the story, before you even think about who they are at the start. Are they courageous and brave? Jaded and unforgiving? Heartbroken? Fulfilled?
And then, move backwards along the line of that state. If they're finishing brave, what does that mean they have to be at the start? Timid? Cowardly? Paralyzed by fear? Aspirational? They want to be a hero, but they don't know how? Still courageous? If they're courageous from start to finish, then why don't they change and how will you show that? Does the type of courage they have change? Does their understanding of it change? Courageous and boisterous and courageous yet humble are two very different things, for example.
And, now that you have your start and your end, it's time to figure out what sorts of things would cause someone to become the thing they need to be. If they're starting at timid and becoming courageous, why are they timid? Because they were bullied as a kid? Then they're going to need to have an opportunity to prove they can get over it. They'll probably also need an event where they confront that past, or something that replicates it, for them to overcome it.
Feel free to draw this out! Make a dot for your end state, then another dot for your inevitable start state, then draw an arc between the two, and map the different types of events they might encounter. If you're just writing as you go and making it up, you still need to know your overall arc or keep it in mind, even if the details are loose.
You'll also need one for each major character whose change you want to showcase. And, to keep things complicated, you may want to understand how they interact with each other, now that you can see their lines. Do you have two people changing to the same thing by the end? That's not super interesting compared to two people who deviate. Do two people have the same character arc? That's not good. Would someone who is timid even get the opportunity to step up if they're paired with someone who is overly courageous? Might that pairing help draw out their weaknesses and change even better?
How are you going to write it? It's up to you. All I can say is, all the best