Each chapter should have one event of consequence. Everything else that happens within a chapter should surround that one event.
Just like with any long-form fiction, approach a chapter as if it were a plot of its own – it has a beginning, middle, and an end. The beginning will always be the same – it is a transition from the previous chapter, which either indicates or redirects the direction in which the rest of the story/chapter is headed.
The middle and end of a chapter is where you have options. One way of going about it is that you’d have an event take place (middle) before you deal with its aftermath (end), in which case the aftermath is the actual event of consequence. The other approach is to have a consequential event take place (middle), and then the aftermath simply acts as a transition into the next chapter or phase of the story (end).
Let’s break it down.
A chapter following the first approach might go something like what follows. Jenny suggests to Caitlyn that they should throw a party so that they can have one last hurrah before college starts (beginning part one). In the days that pass, they plan the party and Jenny invites all their friends, then she tells Caitlyn she’s got everything covered and not to worry (beginning part two). On the Friday night that follows, Caitlyn is surprised to see that everyone in their grade turns up to her house expecting a party and Jenny is nowhere to be found, implying that Jenny made a fool out of her after telling her that it would be a small party at Jenny’s (middle). Caitlyn sends a hundred irritated people home, then finds Jenny relaxing at her own house and she confronts her; the argument ends in a revelation of a bitter truth about their friendship – one that changes everything (end).
Readers are led to believe that the party will be consequential in some way, but it’s the aftermath that serves as the main event because it changes the course of the story and causes a reaction from the protagonist, Caitlyn, which would serve as the opener for the next chapter and possibly shape the way the rest of the story plays out.
The other approach goes a little differently. The beginning would comprise of the decision to have a party, and its subsequent planning; the middle would see Jenny and Caitlyn’s confrontation at the party; and the end would merely be the calm *after* the storm, when Caitlyn’s feelings are addressed and she indicates that a big change is imminent, and soon.
That’s generally how each chapter of any book goes. The plot of the book is moved forward by the expansion of a set of story beats that lead up to bigger plot points (wherein the stakes are heightened).
Now let’s address the character development aspect. Obviously, characters have to see some growth or disintegration during the span of every chapter. The way to show that is through actions and events that consist of beats and plot points. Always remember to add external, as well as internal, consequences to every action; that means that the scenes are written in such a way that it is clear for the reader to notice the stakes, potential gains and losses, and point of every action – nothing happens just because.
The reason I mention character development last is because if you’re writing effectively, then the events of every chapter are already developing characters for you. You know they say it’s not what happens to you that makes up who you are, it’s the way you react and respond to an event that defines you. Always apply that concept as you write through a chapter. The actions that your protagonist takes in response to whatever happens to them in their fictional life is how the readers will define them.
If Caitlyn plans her revenge on Jenny, then she’s a pretty immature and emotional girl who is about to waste a heck of a lot of time; she’s not that wise, but she’s a teenager so we’ll tolerate it. We can also tell that the story will be about overall disintegration of either one or both girls.
However, if she decides to let it go and goes off to college for a fresh start, we’ll know that what Jenny did to her was a learning experience that will somehow affect her future relationships because she’ll have trust issues. No doubt, the story will be about overall growth of her character.
Actions speak louder than words ever will, so focus on making the actions your characters take meaningful and consequential. (This goes without saying, but make sure they are actually taking action, and constantly. Avoid deadbeat characters!) If your characters are completely inside themselves and barely proactive, growth cannot occur; you know, because growth does not occur within a bubble.
We learn from interactions with things and people, so make interactions happen and make them count!