The patented Duvall grin when Vito mocks Johnny.

The look of suspicion on Sollozzo's face when Mike asks to go to the toilet.

The look on Appollonia's face when she stares at Michael while admiring (more, IMHO) her gold necklaces.

The embarrassed look on Sonny's face when Sandy tells him to watch himself at the wedding (it's a combo of embarrassment and "oh yeah, I'm about to work Willie to death on one of these bridesmaids.")

The look on Mike's face as he violently grips the newspaper wondering if his father is dead or alive; you KNOW he wants to kick someone's ass right there and now.

When Vito stares out of the window at Mike and Kay arriving at the wedding; there's such love and warmth and maybe some sadness in those eyes.