It's also called Character Development.

Continuing on from last post…
If tension is what drives your story, then the fuel is change.
… FUCK guys I don’t even—everything changes, EVERYTHING, EVER, IN THE WORLD. Do you not understand this?
Well no, I mean, most of you beginners are like twelve and you’re too young to notice how much things change, I mean…
No wait, what am I saying, JUST LOOK AT YOURSELVES. Idiots. Have you not changed since you were a child? And for those of you who actually ARE twelve, you’ll notice that that wasn’t all that long ago.
Things change. People change. Even the mountains, even the sky, even if you can’t notice it. Always.

So why the HELL do I keep seeing shitty stories where all sorts of shit happens—stuff blows up, characters die, mass genocide—and at the end, the main character is the same damn person he was at the beginning? He’s no wiser, no more or less jaded, nothing.
Fuck, how does that even happen?

When you write, you are documenting a CHANGE. Maybe even a shit ton of changes.
Let’s look at an example: Sailor Moon.
“But wait,” you might say, “Sailor Moon may be TOTES AWESOME, but it’s not a good example, is it? After all, it’s all about the sloppy writing and static characters.”
It kind of is. But Usagi, the main character, changes a lot over the series. It goes something like this:

Wimp ==> Whiny warrior who beats the bad guys but still wishes for a normal life ==> Brave warrior who realizes how great her powers are because they mean she can protect her peeps ==> FUCKING MESSIAH kind of ==> Ass-kicking peacemaker who realizes the futility of war, decides that violence won’t save her problems, and saves the universe with love.

It’s even more pronounced when you actually watch it. Especially in the manga.

Now, if a fluffy kids show can manage some friggin’ character development, WHY CAN’T YOU?

The things that happen, the people they meet, their own actions—these things all change a person. Sometimes it’s for better, sometimes for worse. Sometimes it’s neither better nor worse, simply different. Either way, they’ve got to change.

This is mandatory for a protagonist (That’s what makes them a protagonist), but don’t forget that other characters can change too! Of course, there’s no way every character is going to change much—that could pull too much attention away from the focus of the story, after al—but try to do as well as you can.