Two favorites....Brando ad libbing with the cat in the opening scene, and later Brando, bracing himself for Tom's delivery of the news about Sonny telling him, "...but youhad to have a drink first....well, now you've had your drink....."
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
