This is an action packed book, and characters' actions really define who they are as people. Jerry's defiant act – refusing to sell chocolates – makes him seem heroic. Everything we see Brother Leon screams: "Villain!." Emile Janza's brutal attacks on Jerry make us fear to walk the streets, and he's only in high school.
Some acts are more ambiguous – we aren't entirely sure what they tell us about the character. For example, The Goober unscrews all the screws in Room Nineteen, but he doesn't want to. Does this make him seem weak and spineless, or does it make him seem like a victim? In between the two? Something else entirely? You decide.
Thoughts and Opinions
The thoughts and opinions of the characters in The Chocolate War tell us things we might not get from their actions, speech, or from the minds of others. For example, Jerry's thoughts reveal his sadness and isolation over his mother's death. They also reveal that he wants to escape the boring life he sees his father living and to protect his father from pain and harm. Knowing these things helps us understand some of the deeper motivations for his actions in the novel.
We get lots of direct characterization, often in the form of a thought or opinion. For example, Jerry characterizes Emile as an "an animal" (7.6) who will hurt anybody. Similarly, Obie characterizes Archie as "brillian[t]" and full of "cruelties" (2.16). Readers have to decide whether the direct characterizations we receive are accurate, or not. We might disagree with Obie about Archie's brilliance, even as we agree that Archie's cruel. We might reconsider Archie's characterization of Emile as an animal, when we hear how Emile feels about being called one. Or maybe not.
Speech and Dialogue
This one is especially important in terms of Brother Leon, since we can't see inside his head. When we hear his slimily articulate speech, there's little doubt that he's a thoroughly nasty guy, who uses speech to abuse others.