A guess is that Brando, trying to get into character, imitated the raspy voice of Frank Costello. He was the only Mafia Don of his era to appear on televison, during the Kefauver Subcommittee hearings (1950-51). The heariongs were viewed by tens of millions of Americans; and, although Costello's face was not shown (he permitted only his hands to be televised), he was on camera for hours.