Kerouac is extremely skilled at reflecting differences in character with differences in speech, probably because he had so much real-life material with which to work. Compare a typical Cody passage:
"Been waitin for ya old buddy, come on down right away, but I'll be goin to work at midnight so hurry up and you can visit me at work soon's the boss leaves round two and I'll show you my new job of tire recappin and see if you cant bring a little somethin like a girl or sumptin, just kiddin, come on down pal... " (12.1)
To the words of a character like Monsanto:
"This is the kind of place where a person should really be alone […]. When youbring a big gang here it somehow desecrates it […]. There's such a sad sweetness to those trees." (18.3)
Cody is frantic, rushed, and energetic, while Monsanto is measured, reflective, and calm. You get the idea.
Jack chooses a few specific actions through which to examine the character of his friends. When we say "specific," we mean very particular. The first example is wood chopping.
I realized you can always study the character of a man by the way he chops wood. Monsanto […] showed us how he conducted his whole life […] by the way he took neat little short handled chops […] getting his work done in reasonably short time without too much sweat […] – Whereas old Fagan pipe-in-mouth slogged away […], also getting his job done, silently, not a word – But Cody's fantastic fiery character showed in the way he went at the log with horrible force, when he brought down the axe with all his might […] you could hear the whole tree-trunk groaning […] – Nevertheless it took him longer and much more sweat than Monsanto. […] It was like an example of vast but senseless strength, a picture of poor Cody's life. (21.5)
And example number two? Driving, of course – we would expect nothing less from the man who spent a decade on the road. Kerouac examines driving habits as a way to compare Cody to Dave.
You feel safer with Dave Wain," he writes. "Cody gives you a sense of dooming boom as he pushes the night out […] not because he'll lose perfect control of the car but [because] you feel the car will take off suddenly up to Heaven. […] With Dave Wain it's all conversation and smooth sailing, with Cody it's a crisis about to get worse. (24.1)