One interesting thing, though: our narrator doesn't do much traveling into characters' minds. He tells us occasionally that a character "frowned as he thought" (2) or "thought for a moment," but he never actually tells us what the character thinks.
Instead, all our insight into these characters comes from description. George lays his cards down "thoughtfully" (2); Lennie's eyes are "frightened" (2); Curley's wife speaks "playfully" (2). It's almost as though Steinbeck is deliberately withholding insider information—maybe as a mirror to the characters in the play.
In this hard-hitting, straight-shooting story, it simply wouldn't be fitting to have a narrator gushing about how everyone feels all the time. Without personal commentary or much narrative insight, actions and speech do the work of exposing characters.