What do we mean by straightforward? We mean that Steinbeck doesn't beat around the bush. Subtlety is not his game, especially when it comes to characterization. Steinbeck doesn't show you how a character is feeling by telling you about their body language or facial expression or actions: he just straight up tells you. Look at this:
Charles had one great quality. He was never sorry—ever. (3.3.5)
Okay, so Charles = never sorry. Got it. Now we know something about Charles. Or check out this passage about the tension in Kate's whorehouse:
Kate knew how the girls felt about her. They were desperately afraid of her. She kept them that way. It was probable that they hated her, and that didn't matter either. But they trusted her, and that did matter. (40.3.4)
This passage is a little different from the first in that it doesn't directly tell us something about Kate's personality, but it does tell us something about Kate through the way she runs her house: that she is powerful and scary. And the language Steinbeck uses to tell us is super blunt: "They were desperately afraid of her." You don't get much more straightforward than that.
So why write this way? Since the story spans a period of about fifty years, Steinbeck can cover a lot of ground by giving us these kinds of summarizing descriptions. And they pack a punch too. "He was never sorry—ever" is at once telling us something very simple but also really unnerving. What kind of person is never ever sorry? What does this mean about how Charles is going to behave in the future? Those five words give us a lot to think about.