Sal notices the way people drink. He notices when they drink, why they drink, how they act when they drink, and what they drink. This tells us something about the characters. Maybe not anything vital or central, but it gives us a better idea of who they are. For example, we would prefer to know ahead of time that, when drunk, Stan can’t be dragged away from women. It also doesn’t hurt to hear that Slim will talk your ear off once he’s had a few shots. The fact that Sal notices isn’t completely unimportant, either.
This mostly pertains to Bull and Jane Lee. Drug use is their defining characteristic. Sal goes into great detail about Bull and Jane's past, and their, um, recreational habits tell us a lot about their lifestyle and personas.
The way that different characters approach sex is an important part of who they are. Dean, for instance, is nearly defined by his lusting after every female that moves. Sal, on the other hand, differs from Dean in his need to connect soulfully with women. Marylou is characterized by her sexual adventurousness (she was up for Dean’s voyeurism, remember?).
Because Sal is so bent on geography, and because he’s telling the story, it matters where people are from. Dean wouldn’t be Dean is he wasn’t a hero of the West. Sal imposes his ideas about the different parts of America onto the people he meets. And that makes it…a tool of characterization.
Slim is basically defined by his "orooni’ language. That’s the whole point of his character. And aside from the possibility that Kerouac was sitting around going, "I love the word ‘orooni’ – what’s a fun way get it into my book?" there’s some purpose to it. Slim is barely human. Yes, he’s a person and all, but he has this otherworldly quality to him that is heightened by his otherworldly language. That’s why he’s God, according to Dean. Just as Dean is able to communicate to Victor without language, so Slim is above our every day words. One word expresses it all. All what? All IT. And you thought it was just Dean. But as Sal says, to Slim, the world is just one big "orooni." Why does it excite Dean so much? Because "orooni" captures the impossible to express nature of his IT, the thing he's searching for – the end of the road, the end of his restlessness.
Check out the way Dean speaks in Part I. Slightly mad, yes, but nothing really intense. Then you’ve got Dean in Part II, slightly maniacal and delightfully odd. Fast forward to Mexico and finally to Part V, when the man can barely finish his sentences, and it looks like we’ve got some "speech as a tool for characterization" going on.