In my opinion, every character in a story is an externalization or projection from the author. Where else do characters come from, if not from the author's persona / personality?

The protagonist arises from the writer's dominant self. The antagonist from the writer's shadow self, etc.

So what does it mean that a character's action / choice comes from the writer and not from the character?

It means that the writer is not communing with the writer's own inner core. The writer is instead being driven by the (external) plot or by some plot contrivance or by some guru's advice instead of by some inner need of the writer.

This kind of misdirection also tends to result in phony or on-the-nose dialogue. Or plot holes that strain credulity. Or many other flaws that weaken a story's effectiveness.

The first step is for the writer to get in touch with her or his inner self / needs.

If the writer can't do that, then the script is doomed.

The second step is to shape the inner story into an external story that makes sense for the audience.

This is what the craft teaches us how to do.

Step 1: Honesty.

Step 2: Hard work / craft.

One without the other means that the script is DOA.