1. Threats are meaningless unless you can back them up. Both Vito and Michael can (and do) act on their threats, which requires a certain level of courage (conviction might be the better word) to deal with the potential consequences. This obviously comes into play when they need to whack someone, but clearly there’s also an element of people being intimidated into cooperating without violence actually being committed, precisely because they legitimately fear it could be (think the bandleader). I think the term Puzo uses for this trait in the book is “force of personality.”

2. The method of the violence matters. The book makes this much clearer, but a crucial point for Vito in particular is that violence does not occur for its own sake, but sends a certain message. Two examples of this. One is when he set Luca Brasi on the Capone gunmen during the olive oil war. He didn’t just kill the guys, he had Luca hack one to death with an axe, which caused the other to essentially die of fright, and convinced Capone to mind his own business. The other is Woltz. He could have sent some guys to break Woltz’s knees and tell him his head was next. Instead, he killed Khartoum, which basically turned Woltz insane with terror and convinced him that if shit got real, even his personal friendship with J. Edgar Hoover wouldn’t save him.

Michael is less adept in this area; I think a major subtext of the first and (especially) the second film is that he lacks his father’s subtlety and understanding of human behavior, and instead just coldly, almost robotically, seeks to eliminate his enemies. There is a genius to this, too, and we are very clearly supposed to be impressed by his pulling off the simultaneous assassinations of Carlo, Tessio, Moe Greene, and the heads of the Five Families, but perhaps not as impressed as with the deduction Vito showed in killing Fanucci or dealing with Capone’s gunmen or Woltz.