In reading the novel again, I noticed some things.

Despite having been in the Army or at Dartmouth since just after Pearl Harbor, Mike knows that the "house seemed to be full of men he didn't know" which may be true enough, but what are we (and he) to expect? He's been gone almost 4 years. However, Puzo also had Michael acknowledge that those men guarding Vito's house "were not of Clemenza's people." How'd he figure that?

Michael elects to stay in his father's office with Sonny, Tessio, Tom, and Clemenza when Sonny tells him "You hang around me Mike, your gonna hear things you don't wanta hear." However, in chapter 9, Puzo has Mike complaining that he's being enmeshed in family business "against his will" and he resented Sonny using him to answer the phone.