on Writers Helping Writers:

If you’ve researched story structure at all, you know there are many models out there, and they’re all slightly different. The most popular forms tend to follow the three-act structure, which resonates with many readers regardless of genre or format.

Act 1 sets things up for readers by establishing the protagonist, their story goal, the setting, and all the basics.
Act 2 builds on that information, introducing escalating conflicts (both internal and external) that block the character from their objective.
Act 3 resolves the story conflict in a showdown that determines whether the protagonist succeeds or fails at achieving their goal.

Within this simple framework, certain events need to happen not only to progress the plot, but also to encourage the character to become more self-aware, make positive internal progress, overcome setbacks, and so on. This journey is essential if your character is to progress realistically from Once upon a time to The End. It’s not an easy path, though, and sometimes characters balk; they’d rather stay where it’s comfortable and safe, thank you very much. The status quo may be stagnant or even unhealthy, but it’s what they know.

But a stalled character means a stalled story—which is death for reader engagement. At times like these, your protagonist needs a nudge (or a full-fledged shove) to reach the next important story event. This is where amplifiers come in.