Simple, Sparse
Steinbeck doesn't like to beat around the bush. He tells it like it is—no frills, and no nonsense.
The dialogue between the characters is oftentimes direct and to the point. There's no time for long, breathtaking soliloquies when there is work to be done:
"Now he'll feel better," Billy assured him. "That yellow poison is what made him sick."
Jody looked unbelievingly at Billy Buck. "He's awful sick."
[…] "Yes, he's pretty sick." (1.154-1.156)
Sure, these are simple folks living simple lives, but that doesn't mean they're not complicated on the inside. They may use few words, and Steinbeck may use few words to describe them, but he packs a whole lot of meaning into those few words.
Take, for example, when Jody's mom seems shocked that Jody only wants a lemon for his Grandfather, and not one for himself, too. She just says, "Jody! You're sick!" (4.169).
But we know he's not sick. We know he's feeling a little bit sad that westering is over. We know he feels bad for his Grandfather, whose best days are behind him. And we know that he's trying to do something nice, something selfless for the old guy, and that's why he doesn't want a lemon for himself. And we know his mother knows not a lick of this.
Does Steinbeck bother telling us any of this? Nope. He just uses a sparse style, and leaves the sussing out up to us.