In the deleted scene when Sonny goes into the Don's Office, after the Don is shot, Sonny stares at the Don's empty chair, and then instead of sitting in the Don's chair he sits in a chair on the side of the Don's desk.
This, to me, was symbolic in a couple of ways.
Sonny realized that the S**t had really hit the fan. He realized that there was a strng possibility that the Don would die. And I believe that his staring at the empty Don's chair and not sitting in it symbolized that deep down inside, he knew that he would never be the kind of Don that Vito was. Sonny choosing not to sit in that chair also symbolized the great respect that he had for his father. But he really knew that he could never fill the Don's shoes.
Don Cardi
