I recently reread The Godfather for the first time since I was about 14. Mario Puzo is no prose stylist -- the book reads like a very long magazine article rather than a novel, and the sections on Lucy Mancini (the girl who Sonny bangs at the wedding -- she is actually a major character in the book) and her gynecologist (not in the film at all, thankfully) are totally pointless and should have been cut. But the book does give a lot of detail that simply couldn't fit into the film, and gives some insight into the motivations and especially the strategies of the characters.
Aside from Lucy Mancini and her abortionist, the film also de-emphasizes Johnny Fontane. People may not realize it today, but a key factor in the novel's success was that readers actually got to see
longstanding rumors about Sinatra in print, albeit concerning one "Johnny Fontane". Keeping the Fontane story would have made the movie at least 4 hours long -- and one Francis Albert from Hoboken might very well have hired various consiglieri to block the production. So the Fontane plot was mostly discarded
They did keep the horse's head scene though. That entire section and the Fontane character could be cut from the film with minimal damage to the plot -- but the horse's head had been the most talked about scene in the book, so TPTB decided to retain it.
The important difference between the book and the film -- aside from Coppola being a stylish director and Puzo being a less than stylish writer -- is the young Michael. In the book he's already fairly
hardened and cynical from the outset.
Here is an excerpt from the novel, Michael's big speech on whether to kill Solozzo and McCluskey:
“You shouldn’t let that broken jaw influence you,” Hagen said.
“McCluskey is a stupid man and it was business, not personal.”
For the second time he saw Michael Corleone’s face freeze into a mask
that resembled uncannily the Don’s. “Tom, don’t let anybody kid you.
It’s all personal, every bit of business. Every piece of shit every
man has to eat every day of his life is personal. They call it
business. OK. But it’s personal as hell. You know where I learned
that from? The Don. My old man. The Godfather. If a bolt of
lightning hit a friend of his the old man would take it personal. He
took my going into the Marines personal. That’s what makes him great.
The Great Don. He takes everything personal. Like God. He knows
every feather that falls from the tail of a sparrow or however the
hell it goes. Right? And you know something? Accidents don’t happen
to people who take accidents as a personal insult. So I came late,
OK, but I’m coming all the way. Damn right, I take that broken jaw
personal; damn right, I take Solozzo trying to kill my father
This is the COMPLETE OPPOSITE of Michael's attitude in the film ("It's not personal Sonny -- it's strictly business"). Presumably Coppola made the change -- and it's a change for the better.
The film softened the character of Michael, which made his story arc more interesting. Indeed the film softened most of the characters. Luca Brasi in the movie is almost a comic figure, clumsily practicing his speech. The novel's Luca is a monster -- you won't soon forget
what happens to his Irish girlfriend.
Here's a page from the book marked up by Coppola:
From the set (note Brando's practice of using cue cards - reminders or part of his method?):
Potential casting list from Coppola. Robert Evans was opposed to Pacino and there was a huge battle over the casting of that role. The studio wanted... Robert Redford (?) or another name actor. Difficult to imagine anyone other than perhaps DeNiro in that role: