Psychological Doubling
Pairing the main character with a second character who mirrors them. The double shows what the hero wants, fears or secretly is.
Amadeus
Salieri and Mozart are built as doubles. Salieri has the piety and discipline Mozart lacks, and Mozart has the divine gift Salieri desperately wants. Each is the other's nightmare. In Mozart's music, Salieri hears the voice of God, which is also the voice that has rejected him. I feel like this mirroring is the whole psychology of the film.
The Power of the Dog
Phil and Peter are each other's shadow. Phil puts on an exaggerated toughness to hide his own nature. Peter hides his intelligence behind a fragile manner. Each sees something in the other that neither can name. I feel like the genius of the film is that you notice they are doubles before they do, and when they do recognize it, the consequences are catastrophic.

