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James is one of those supporting characters who becomes significant because he's a truth-teller. He's not as bold as Lurhetta Outlaw (more on her elsewhere in this section), but he similarly refuses to back down from his truth. In his case, the truth also happens to be reality: That spoil-heap will crash down on the Higgins family if they don't leave soon.
Here's how he handles telling that truth to Jones, who really doesn't want to hear it:
James Lewis raced one hand through his crisp, graying hair, as though his scalp itched. He cleared his throat. "I don't believe it will slide all the way," he said cautiously. "The grade of that slope will be too steep." And then, politely: "I'm afraid there will be a momentum and a pressure that will bring it all crashing down." And then he folded his hands, looking worriedly at Jones. (6.118)
Sure, Jones still won't listen to him, but M.C. does. That's because James speaks with reason (not to mention, it doesn't hurt that he's just a really nice, polite person).
In fact, he's pretty much the opposite of Jones. James is reasonable, humble (he's "just a collector" (6.46)), and kind. That kind of makes Jones look bad, doesn't it? But that's what a supporting character is supposed to do: bring other characters' qualities into broad daylight.