The novel was a huge success before the film came out, and made a giant impact. So people were all leaped up about the film. Most of the initial buzz was over Brando's performance. His lines (such as "I'll make him an offer he can't refuse") got into the popular lexicon overnight. In fact, Brando's performance tended to overshadow Pacino's and Duvall's. Only after a period of months did people begin to appreciate what an overall triumph the film was. But before that, and after, Brando's Vito Corleone was more popular than the president of the US.