Brando really caught the essence of Vito Corleone when he learned of Sonny's death.

He allows himself a few seconds of grief (witnessed only by Hagen),then almost immediately his demeanor changes,and he commands that no inquiries are to be made,and no acts of vengeance taken.
He then requests a meeting with the other Dons,and leaves the room, a father without a son.

Vito went from being a heartbroken Dad back to being Don Corleone.
Such an amazing transformation in only a few seconds,simply by using some carefully applied facial expressions and body language.
.
An incredible performance indeed !!