Once you decide what the overarching umbrella theme of your story is, then make sure that theme is abundantly clear within the A story and the protagonist’s character arc, but also present within the B stories and the other character arcs.
I was told once to write out the theme on a post it and stick that postie on the computer while writing, so the theme will be present within every scene of the movie – not only within the major story beats but felt even at the scene level. This will help to push your scenes in the right direction and reduce the possibility of confusion.
Okay, so let’s throw out some ideas to see how this might translate to the writing.
Let’s say your theme is redemption. Then your protagonist should be on a journey that takes him from being a man, ruined, without integrity (cheater, liar, asshole, narcissist, etc.) to learning that he can no longer live that way. He’s a fuck up. He’s fucked everything up with his wife, his kids, his job, his friends – basically, he’s a massive fuck up in life. The journey of the story has to teach him how to not be such a fuck up. He finds integrity throughout the journey of the story. Or, in some cases, if you’re writing a negative character arc, like The Wrestler, not. He’s confronted with change (he almost died from his heart attack and might still die) but decides not to change, and that’s his journey. However, that was still a story of redemption and supplication. He was his own nemesis.
So, in this event, the A story – an external goal that the protagonist wants to accomplish – must be teaching him about the meaning of redemption. What it would mean to not be a fuck up – and what it would look like if he were capable of living his life with integrity. That A plotting and the emotional journey/character arc will be abundantly clear.
However, say the character has a daughter. The B story with the daughter (both in terms of the plotting and the emotional journey) should also touch upon the theme of redemption, which will underscore the protagonist’s journey and A story. The daughter’s character arc should also be one of redemption, or perhaps a companion arc that will serve to underscore the arc of the protagonist. In The Wrestler, it was a negative arc – the daughter was not redeemed – which foreshadowed the negative story and character arc of the film.
If there were a wife character, or business partner, or other characters within the story, the stronger choice will be to have their B stories and character arcs dovetail with the A story in a way that both informs and enlightens the theme of the movie. Their individual arcs don’t have to be one of pure redemption, but if not, then their journeys should somehow inform the protagonist’s journey in a way where we learn something from that journey. The combination of paths should make the piece stronger overall.