Steinbeck's style is methodical. In the use of non-narrative chapters to establish and develop themes, Steinbeck carefully constructs a context for his narrative that deepens the social commentary of the novel. These non-narrative chapters clearly demonstrate a method at work.
The cast of characters is also carefully crafted with repetition in key places. The road-side stops on the way to California often include an out-cast figure or a frightened man who has turned to bitter judgement. These qualities are sometimes found in the same person (i.e., the one-eyed man and the fat gas station attendant).
While certain other moments in the text may present some lapses in planning, we should note that this is a large novel. The most pronounced lapses in the text come in the silence of many of the characters (Casy, Granma, Uncle John) for extended periods of time. However, if these characters were continually included in the family's conversations as Tom, Ma, Al and Pa are included the novel would need to be longer, as many of the conversations would turn into "round-table discussions" (of which there are a few in the novel already).