Pacino was better in II
Duvall was better in I
Shire was better in II(and even better in III)
Cazale was great in both.
DeNiro did an outtanding job of inhabiting the character Brando created without doing an impersonation. It was a difficult bit of acting and IMHO one of the best performances in the trilogy.
"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!"
"I stood in the courtroom like a fool."
"I am Constanza: Lord of the idiots."
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