Why one right choice at the end doesn't prove your theme
A protagonist's climactic choice on its own is consistent with your story's position without ever establishing it, because a second position would explain the same choice equally well.
Why theme lives between characters, not inside one
What a reader experiences is not a character but the space between two of them, so the same claim carried by three different pairs is what stops a theme sounding like one note.
You can build every character in your cast correctly and still end up with a flat story.
That's because what your reader feels most isn't any one character. It's the relationship between two of them.
In Frozen, Anna and Elsa want the same thing, and it comes out as two kinds of love that can't fit together. Neither sister carries the story's argument alone. Their relationship does.
So one pair can carry your point through a fight. Another through sacrifice. Another through two people quietly standing by each other.
That's how people learn in real life too. The same lesson lands differently from someone who fights you, someone who sacrifices for you, and someone who just stands by you. You need all three.
And one fact can rearrange every relationship at once. When Hans reveals what he actually wants in Frozen, every alliance changes, and nobody's earlier behavior has to change at all.
So list your pairs, not your characters. Then check which pairs carry your point, and which of those three ways of carrying it you haven't written yet.
How to show your theme without characters preaching
A position can be stated, embodied, or carried by what happens to somebody, and a cast can be fully staffed on the first and starved on the other two.