Symbolism, Imagery, Allegory
When Pyle tries to lie, it's obvious. He's not practiced in it. He has no ear for it. His straight talk, and his expectation that people are being straight with him, reveals Pyle's innocence about the world.
When Pyle speaks, he bears his soul to you, but he also shows just how deceived his soul is about who you are and what you're about. Just look at what he says to Fowler, Phuong's lover, about falling for the girl:
I had to tell you—I've fallen in love with Phuong. (1.4.2.28)
He expects that Fowler will use this information honorably, as he himself is being honorable by revealing his intentions, but Fowler's like, dude, people don't do that. They sneak around and take what they want (1.4.2.33). It doesn't register with Pyle.
This is why Pyle can come to trust a murderous bandit with deadly explosives. This is why he comes to Fowler for help proposing to Phuong. Pyle's speech perfectly reflects his ideas, but his ideas have little connection to the real world.