I read in the IMDb trivia the hug was ad-libbed by James Dean on instinct. Well, it certainly worked! It was so real, so heartbreaking. Very powerful scene.
‘Six inches is perfectly adequate; more is vulgar!' (Prime of Miss Jean Brodie Re: an open window).
Those taken-aback expressions of Raymond Massey as Cal's (James Dean's) father, Adam Trask, are genuine! Dean's "method acting" and Massey's "old guard" approach made the two of them constantly clash, both on and offscreen.
It's a tribute to Elia Kazan's directorial savvy and skill that he could capitalize on the mutual opposition between Dean and Massey and make it work so beautifully onscreen. (Of course, neither Dean nor Massey were exactly chopped liver, either!)
The movie is wonderful, but it's really a seperate entity from the book; Steinbeck's Adam Trask isn't quite as implacably sanctimonious and self-righteous as Kazan's Trask is.