How we cite our quotes: All quotations are from The Usual Suspects.
Quote #4
VERBAL: Who is Keyser Söze? He is supposed to be Turkish. Some say his father was German. Nobody believed he was real. Nobody ever saw him or knew anybody that ever worked directly for him, but to hear Kobayashi tell it, anybody could have worked for Söze. You never knew. That was his power… And like that, poof. He's gone.
Söze is so mysterious he seems far beyond our experience—like he exists on this higher, transcendent level of evil. Of course, the irony here is that Verbal is Keyser Söze—he's a normal-seeming dude who looks like Kevin Spacey.
Quote #5
VERBAL: The greatest trick the Devil ever pulled was convincing the world he didn't exist.
This famous line actually comes from the great French poet, Charles Baudelaire—it's a quote. If you don't believe in the devil, Verbal implies, you're ready to be manipulated by the devil, because you won't be on guard against him. This is the same way Kujan relates to Keyser Soze: he doesn't believe he's real, and hence is ready to be manipulated by him—which is exactly what happens.
Quote #6
VERBAL: To a cop, the explanation is never that complicated. It's always simple. There's no mystery to the street, no arch criminal behind it all. If you got a dead body and you think his brother did it, you're gonna find out you're right.
Verbal is basically describing the way Kujan thinks. Kujan doesn't believe in the myth of Söze, and, ironically, by telling him this story, Verbal convinces Kujan that Söze isn't real, and that Dean Keaton is the main bad guy. Of course, this is what Verbal intended to do, since he (Verbal) is actually Söze.