I am with the naysayers here. Duvall was Hagen. Period.
A good example of not using an actor to reprise another actor's role in the seies was the inabiity to have Clemenza come back because of contractual disputes. Instead of getting another actor to "play" CLemenza, FFC and MP "killed" Clemenza off and made someone else his successor. What that did was to create one of the greatest roles in the trilogy, Frank Pentangeli.
"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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