It's important to remember what the ward was like BEFORE Mack showed up: it was a quiet, humorless place. Men were afraid to laugh, to joke...heck, under Nurse Ratched's iron fist, they were barely comfortable talking at all!
Enter McMurphy. Cracking jokes, having fun (gasp!), and generally stirring things up, especially in the subtle and not-so-subtle ways he defies Nurse Ratched, he has a profound effect on the other men. They are empowered to remember what it was like to be, well, men (or even just human beings), and then act that way.
Harding's hands are described as fluttering and birdlike. Mack's are more rough and worn. I don't want to give away the story here...it should be pretty obvious what this says about each man.
Good luck! Hope you enjoy the rest of the book; it is one of my favorites.