John Gavin:
I seem to be in the minority here (except for Mr Hitch) but I found his opening scene to lack nuance; one-tone of discontent running through his lines. Hey, I'll never be as successful and famous as him, and dont' try, but acting classes demonstrate how you can go "against" the lines, within reason, You could debate that a man who is so fully discontent would not break away from that mood; but in a film, you need some spark of what-will-he-do -next, as an actor.
For example, when he says "tell you what, you can lick the stamps" (and some of the other), he could had actually ended that on a deprecating amusing gesture or half-hearted laugh to show his vulnerability and loss of hope, especially when he's with with his lover and his guard is down. Another technique for Gavin would be to fully boil over with frustration, then catch himself and calm down to a sense of . His scenes with Perkins were more straightforward, so his uptight nature fit.