Steinbeck's writing style mirrors his characters. Of course the author writes as the men would literally speak, but on a deeper level, the language of the book is simple but compelling—just like the characters. Take the very first sentence: "A few miles south of Soledad, the Salinas River drops in close to the hillside bank and runs deep and green" (1.1) Straightforward, descriptive, and honest: just like the book.
Though the characters never gush about each other, it's clear that they feel deeply. For example, while George explains that he sticks with Lennie because "you get used to goin' around with a guy an' you can't get rid of him," what he's really saying is that their friendship is the only thing he's ever really had to hold on to. It's more "still waters run deep" than "OMG you're my BFFL.
Again, Steinbeck uses his writing style as another means to suggest that every story is important, no matter whose story it is. Though these characters are workers without access to big vocabularies or grand philosophies, they can still communicate about the things that really matter.