I think thats a fair statement Apple. One could make the argument that they were easy to figure out. But I think all these characters as in most good stories have character traits that are not clearly defined in the story. There is always room for the reader to 'read between the lines.' Most good stories hardly EVER tell the audience every minute detail of the narrative and characters. It leaves atleast a little room for interpretation. Considering these are two main characters and considering it is Puzo and FFC, I don't think this story is an exception to that rule of good story telling.

(The book offers lots of details about Sonny and Fredo that the movie leaves out.)