Phil Parker
A-STORY VS. B-STORY: the thematic mirror

Audiences go to films for two primary reasons: 1. To be entertained. 2. To care.
How do screenwriters convince an audience to care? Assuming the ending is a positive one, they often do it by:
1. Giving the main character a relatable, emotional wound in their past.
2. A wound that has caused the hero to develop a flawed view of the world.
3. That view leads to flawed decision-making and behavior.
Most of that is established in Act One.
4. Then, throughout the story, the hero is forced by the obstacles they face to see that their way of doing things is off-target.
5. They come to realize the wound they carry inside is the cause of their problems.
6. This realization allows them to correct their behavior and achieve their goal (if the ending is positive). The goal may not be exactly what they wanted initially, but it is what they need to fix what was broken within.

Audiences enjoy watching heroes trying to fix their flaws. It gives viewers an unconscious hope that they, too, can fix their flaws one day.
But a film solely focused on a hero performing surgery on their warped psyches would be boring.
That's why the best-told stories create compelling A-plots that are visual and kinetic metaphors for their emotional B-plots. This is called 'thematic mirroring'. It's a way to intertwine the action of the story with the heart of the story. It helps the audience to care whether the hero achieves their goal and what happens to them if they don't.
If you're having trouble with a script, a quick look at some classic/popular films and how their A and B stories reflect the central themes may help.
Check out these examples:

"The Shawshank Redemption" (1994):
In the A-story, Andy Dufresne, a wrongfully convicted inmate, seeks his physical freedom from Shawshank State Penitentiary. The B-story revolves around Andy's inner struggle for emotional freedom from his despair over a cheating wife and the degradation of unjust imprisonment. The thematic mirroring is evident as Andy's escape from prison represents his ultimate liberation from the emotional chains that have held him captive.

"The Godfather" (1972):
The A-story centers around Michael Corleone's rise to power within his family's organized crime empire. The B-story explores Michael's internal conflict as he grapples with the corrupting influence of his family's legacy and his own moral choices. The thematic mirroring is seen as Michael's external pursuit of power mirrors his internal descent into darkness and moral ambiguity.

"Rocky" (1976):
The A-story follows Rocky Balboa's training and journey to compete against heavyweight champion Apollo Creed. The B-story explores Rocky's personal growth and self-belief as he discovers his own worth and fights against his self-doubt. Thematic mirroring can be observed as Rocky's physical boxing matches serve as a metaphor for his inner battles and determination to prove himself.

"Taken" (2008):
In the A-story, Bryan Mills embarks on a relentless mission to rescue his daughter, Kim, from human traffickers. The B-story centers around Bryan's internal journey to redeem himself for not being around for his family and his desire to rebuild his fractured relationship with Kim. Thematic mirroring is evident as Bryan's external goal of saving Kim parallels his internal need for redemption and the chance to prove himself as a loving and dedicated father.

"Casablanca" (1942):
In the A-story, Rick Blaine holds letters of transit allowing two people to get to America. His former lover Illsa and her current lover, Victor Lazlo, need them to escape the Nazis, but Rick is bitter about how Illsa abandoned him years ago. The B-story focuses on Rick's internal transformation from a spurned and jealous lover to knowing Illsa is better off with Lazlo. Thematic mirroring is present as Rick's external choices and actions in the A-story reflect his internal journey of finding redemption, letting go of past hurts, and ultimately sacrificing his desires for the greater good.