And agency, and agency, and agency. How to get it, how to demonstrate it, whether particular characters have agency, whether they engage the reader in a compelling way.
My good friend C. L. Polk (debut novel Witchmark coming out from Tordotcom in about a year!) once noted that, when encouraging one's self to character growth, it is important to remember that "...the common element in all my failed relationships is... me."
This is also, it turns out, a handy shorthand when it comes to writing, as well.
If one has had a string of failed friendships and broken romantic liaisons, one might do well to look for the cause within one's own behavior and choices and see if one can confront and alter it. If one is writing a character who is flawed in some way (as they generally should be, because flaws and the surmounting or failing to surmount them is one of the core concerns of humanist literature), then the course toward character growth is set the moment that the character is confronted with the thing they keep doing wrong, realizes what it is, and commences the necessarily difficult and soul-searching path of changing themselves and their patterns.
It begins, in other words, in the moment when that character becomes self-aware, and begins to move toward a goal rather than simply acting out a pattern.
This is also, not accidentally, often the moment when the character becomes an engaging protagonist, someone we connect to, hope for, empathize with, and in some cases identify and sympathize with.
Basically, a character becomes a protagonist at the moment when they are confronted with the thing they keep doing wrong, and then have to do it differently this time to thrive, as provoking or being provoked by an epiphany.
I've said on several occasions that one of the major dividing points between "genre" literature and "literary" literature (especially of the Modern school and points north, but dating back to its roots in tragedy and the concept of hubris) is that in "genre" stories, the protagonist usually makes pretty good choices--the best choices of which she is capable and aware--leading him or her to a generally positive outcome. In "literary" stories, the protagonist often makes a series of cascading, terrible choices, and the story achieves its catharsis through that person's stagnation or destruction.
It's possible, of course, for a "literary" novel to have a happy ending (The Color Purple) or for a "genre" novel to have an unhappy ending (The Game of Thrones). There is, in fact, nothing intrinsically superior in one mode over the other, though adolescently tend to assign Deeper Importance to unhappy endings. Tragedy is often deemed More Worthy than comedy, but structurally, as stories, they're just achieving their catharsis in different ways--by modeling aspiration triumph, or by providing gritty release.
Real life does both, after all.
For every person who looks at their life situation and says, "I don't like this; I wonder what I'm doing wrong," there is a person who performs the same operation and responds, "Somebody else caused all these problems and the solution is a new life situation/job/relationship/place/friends." Art has the latitude to reflect and illuminate both--and also to illuminate the fact that, honestly, most of us make both good and bad choices over the course of our lives, and sometimes what seems like one can turn out to be the other.
But in writing the genre protagonist--the protagonist whose motivation is meant to engage, whose agency is meant to provide drive to a story, I think it probably pays for us to remember that simple metric.
"The common element in all her failures is her."
Not because there's no external antagonism. But because she can overcome that external antagonism when she overcomes her own personal failures.
She becomes the protagonist, and begins providing that engine that the genre reader craves, when she recognizes this about herself and begins doing something to change it. This can be outwardly reflected: the fight scene, where the pummeled protagonist, seemingly defeated, stands up one last time and wipes the blood from her mouth and squares her shoulders again. It can be inwardly reflected: the crisis scene where the lover who has hidden her feelings from her beloved finally comes clean, to herself as much as others.
The protagonist who doesn't do these things is the one who, in the parlance of my crit group many years ago, is "failing to protag." Agency is not doing things; it's making choices, and doing things for reasons that demonstrate motive and character and character growth.
Or the failure to grow, for that matter, because that's a real story too.