Duvall's performance is full of subtleties from the beginning to end - I may say any scene with him. For example, when he is looking at Tessio being driven away in the end.
But my favorite scene for subtle playing is that of Mike and Vito in the hospital, when he kisses his hand and says "I'm with you now, Pop." They both are equally perfect, doing the most important conveyance with their looks only.