If you keep in mind that Mario Puzo's The Godfather novel had no literary sequel, I find it astonishing that within the span of less than two years later both he and Francis Coppola were able to produce such a feat!
Nevermind the fact that the idea of cherry picking backstory from Vito Corleone's and forecasting into Michael and the Corleone family future and fates is something to approach with great trepidation for an author and filmmaker. But they pulled it off!
Then they had the audacity and inventiveness to introduce a flashback structure into the film. Coppola told editor Walter Murch if he had only had a little more time editing it before its release that "it might've been great". The resulting 1977 "Novel for Television and its uncensored 1981 Godfather Saga (released only on VHS) is the fulfillment of that wish and evidence of its greatness.
If two great films can be intercut together, reordered (and even have plot lines expanded upon) and still remain seamlessly coherent, that alone is a testament to the genius of both story(s), direction and author(s).
Imagine if you will, a director producing a sequel of the same power a year or two later to any classic work like Stephen King's "Shawshank" or Margaret Mitchell's "Gone with the Wind". Couldn't be done and hasn't. Except in Godfather's case.
(Coppola even managed to direct and write "The Conversation" in between).
Unsurpassed!
