Johnny Tightlips
I'm with you, the scene w/the Don & his sons going to visit Genco should have made the final version. In addition, the scene in which Vito, Tom & Sonny talk about Woltz and sending Luca to 'reason' with Woltz.
IMHO, the two deleted scenes that would have really given Sonny's character more depth were 1) when Sonny calls Tessio requesting 50 men because Sonny wasn't sure who sold Vito out: Paulie or Clemenza 2) when Michael offers to help Sonny & Tessio after Sonny receives the call that it was, in fact, Paulie who sold out the Old Man. I don't know whether I have the exact quote, but Sonny tells Michael: "Even the Old Man will tell you, when it comes to the action, I'm as good as anyone."
Michael: "You have to answer for Santino, Carlo. Ah, that little farce you pulled with my sister. Did you think that could fool a Corleone?"
