At first glance, the novel The Godfather and the movie The Godfather appear quite different. Upon further investigation, however, they do possess some similarities. Although they are different in the areas of the story line, sexual and violent content, and the character reasoning, they are similar in the areas of the characters, scenery, and the detailed conversations.
It seems the major difference between the movie and book is the overall story. Obviously the movie cannot be as long and detailed as the book due to time constraints, but it seems the parts, which were kept, and the parts, which were omitted, worked well with the movie. Some parts, which were omitted from the movie, include the hospitalization and death of the first consigliere, Luca Brasi's detailed infiltration of Sollozo's club and his involvement with a girl from the club, young Vito's upbringing, and the Don's beginning, including his growth of respect he received for doing favors in the neighborhood.
Another difference is the amount of sexual content and violence in the book, which is not in the movie. This may be due to the movie rating systems and again the time restraints involved with making the book a movie. Overall I feel the amount of sex and detailed violence is quite toned down as compared to how it was described in the book. Some examples of this are Sonny's detailed involvement with the bride's maid at Connie's wedding, the detailed beating of the men who hurt Bonasera's daughter, the sexual scene's involving Michael and Kay, and the brutal beatings which Carlo had given Connie.
The character reasoning is another difference between the book and movie. As I read the book I felt I knew why everything happened but if someone had only seen the movie you might not know why a particular character did something. Some examples of this were why was Luca in Sollozo's club at such a bad time but from the book you knew he had careful inspected the club for sometime and was taken by surprise.