I found this great post on Nathan Bransford blog:
WHAT DO YOUR CHARACTERS WANT??
Motivation. It's the powerful emotion that inspires people to get off the couch and grab a tub of ice cream. It's the only thing that is strong enough to pull me out of a very warm bed when it's still dark and cold outside. And it's what inspires Mario to save the princess, despite all sorts of finely rendered cartoon characters standing in his way.
How does this relate to books? Every good book begins with a protagonist who wants something.
I know that this kind of seems obvious (and it probably is), but there's a reason you don't generally see books about characters cast about by the whims of fate without any sense of purpose or desire whatsoever. Even Odysseus, essentially a powerless character blown about by the gods, has a rock solid motivation: he wants to get home.
Now, your character doesn't have to know what he/she wants on page one, but it should be conclusively clear by page 30, preferably earlier. And then, every step your protagonist takes after that point should be a step toward that goal, only they are thwarted at every step by obstacles and characters who have their own set of desires.
Many novels, especially genre novels, have a built-in motivation. Think: "save the princess" fantasy novels. It's built into the plot. The protagonist wants to save the princess. There's your motivation.
But better yet is a novel where a character wants more than one thing, and these two things are at odds. The main character might want to save the princess, but he might just have his eye on the king's throne as well, so he has to decide by the end of the novel which is more important to him. Better still is a character that wants things that are internally contradictory so that they not only have to battle the exterior obstacles to get what they want, but they have to battle conflicting desires within themselves as well.
Here's a way of illustrating that, Super Mario Bros. style.
Good: plumber wants to save the princess.
Better: plumber wants to save the princess while besting green-clad brother with similar goal
Best: plumber wants to save the princess while besting green-clad brother with similar goal, but although he is brave he is plagued by the creeping sense that the gamer controlling his every move might want him dead
Every time you introduce something your character wants, internal or external, whether it's saving the princess, acceptance from their parents, or snaring a white whale, you're introducing aplot arc. The main arc should open at the beginning and close conclusively in the climax of your novel. Smaller arcs may be introduced and closed somewhere in between.
Every single character you introduce, major or minor, should also have their own plot arc(s) with defined goals and motivations. The more important the character the longer and more complex the plot arc(s): i.e. your main villain's plot arc is probably introduced toward the beginning and closed at the end, and we probably have a rather nuanced sense of their own desires and contradictions.
This is often where writers miss opportunities: every character, big or small, has to show motivation, agency, and desire. They have to have their own plot arcs. And it's important that the arcs have a beginning, middle, and end. Unless you're under contract for book two, make sure those plot arcs are closed!
At every step of the way, on every page, with every exchange of dialogue and every action, characters are trying to achieve their desires but run into obstacles, whether internal, external, or because they're encountering characters who want something different than they do. This is conflict.