It's fairly well known that Towne was a script doctor on GF1, and that he wrote the Vito and Michael scene in the garden. The Washington Post mentioned it in a recent interview with Towne:

http://www.washingtonpost.com/wp-dyn/content/article/2006/03/17/AR2006031700457_ 3.html

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His most legendary fix was on "The Godfather." Coppola called in a panic because he was just about to lose Marlon Brando and he still didn't have a scene where Brando and Al Pacino face each other late in the picture, as lord and inheritor and as friends and also as old Jedi to young Jedi. Towne flew in, looked at rushes to get a sense of what was going on and also looked at the cover of the book, which was a stylized version of a hand and marionette strings. He also spoke to Brando because he wanted the actor's ideas.

"Brando said, 'Just once I want Vito not to be inarticulate. He's talking to his son; he's telling the truth; he'd know what he has to say.' "

That night, Towne wrote a speech that nobody has forgotten, inserting it between a couple of Mario Puzo lines: "At that meeting you will be assassinated," and "Remember, whoever brings you the offer will be a traitor." This is what he wrote: "I never wanted this for you. I work my whole life -- I don't apologize -- to take care of my family, and I refused to be a fool, dancing on the string held by all those big shots. I don't apologize -- that's my life -- but I thought that, that when it was your time, that you would be the one to hold the string."