Story Within A Story, Unreliable Narrator
In the same way that Verbal cons Kujan, The Usual Suspects cons the audience—and we mean that in the best possible way.
We're dealing with an unreliable narrator in Verbal, and much of the movie is comprised of the story he's telling us. Kujan doesn't trust him, and we're not sure whether we can trust him either. Kujan thinks Verbal's covering for Keaton, and we (as the audience) go along with that assumption—like Kujan, we don't realize how deep the con really goes.
We spend most of the movie seeing Verbal tell a story that, in the end, turns out to be largely fabricated—maybe a lot of it did happen, but so many crucial details were changed that it's impossible to tell what elements might be true.
And, of course, the biggest change involves the identity of Verbal. Throughout the movie, we believe that the story's protagonist is a real guy, who walks around with a real limp—and then we finally realize that this, the most basic and central element of the narrative, is the center of the con.
Verbal is Keyser Söze, master criminal—and the story we heard was actually Söze's story…and it was designed to fool us.
The Usual Suspects narrative technique is like the devil's technique in Verbal's proverb: "The greatest trick the devil ever pulled was convincing the world that he didn't exist." The movie first convinces us that Söze doesn't exist, before finally flipping the table totally and convincing us that he does—changing the way we view and approach the narrative.