How does Curley cause us to see Lennie's underlying nature?
I would say that there are two distinct moments when Curley brings out Lennie's true nature. It might not be deliberate, but each moment brings out his underlying essence, something that we, as the reader, already knew, but grasp fully in seeing Lennie in these moments. The first would be the fight. Curley is pounding on Lennie and he takes it. Lennie cowers in both pain and fear. This brings out his true nature because it reveals that Lennie is not maliciously aggressive. He is not an instigator, like Curley. Yet, when George commands Lennie to attack him and fight back, we also see Lennie's loyalty to George. At this instant, in following George's words, he catches Curley's fist in his hand and it dwarfs as a result. Steinbeck compares Curley to a fish flopping once Lennie catches his fist. The result is that Lennie breaks Curley's hand, demonstrating that while Lennie has a capacity for love and loyalty, there is also intense destruction within him. The other instant where Curley brings out Lennie's true essence is when he leads the hunting party to catch and kill him. Lennie's underlying essence as a victim, someone who will always be targeted in this life, becomes apparent in this moment, in this instant. Lennie's underlying essence of being afraid and being unwanted is something that is on display, initiated by Curley's own antagonism. The statement and implication is that there are many more cruel and savage people like Curley in the world than the tenderness represented by Lennie. It is a condition that Steinbeck displays, hoping to initiate some level of change.