She acted her ass off:

As Zhao told IndieWire late last year, Buckley was an early favorite for the role of Agnes. “I knew her work. I had a feeling that she wouldn’t be afraid. There was no vanity in her, which is what Fran McDormand had,” the filmmaker told our Anne Thompson. “I’m neurodivergent, and when there’s dissonance, I can’t function. I can’t look at the person. So I need that authenticity, and vanity is the number-one enemy of authenticity. Actors, their greatest blessing they can give to the world is their authenticity and their humanness.”