In The Voyage of the Dawn Treader, we often learn what characters are like based on their actions. For example, when Lucy, Edmund, and Eustace arrive on board the Dawn Treader, Lucy and Edmund see Caspian and "All three shook hands and clapped one another on the back with great delight" (1.31), while Eustace "was crying much harder than any boy of his age has a right to cry when nothing worse than a wetting has happened to him" (1.32).
We'll continue to learn about our characters as the crew of the Dawn Treader faces various situations and each person reacts differently, with Eustace often the weakest or most cowardly. When they are captured by slave traders, Edmund, Lucy, and Reepicheep "sat in the straw and wondered what was happening to Caspian" while Eustace talked "as if everyone except himself was to blame" (3.59). This differentiation will continue until the adventure of Dragon Island, when Eustace goes through several significant changes. After he does, it's again through actions that the narrator characterizes this new Eustace: when confronted with the Sea Serpent, Eustace "did the first brave thing he had ever done" and "began hacking at it with all his might" (8.17).
Clearly, the Eustace who is willing to draw a sword and fight a sea monster is very different from the Eustace who refused to fight a duel with the tiny Reepicheep a few chapters earlier.
Thoughts and Opinions
The narrator of The Voyage of the Dawn Treader has the ability to delve into the thoughts and feelings of nearly all of the book's characters, from protagonists like Eustace and Lucy to relatively minor characters like Drinian. Sometimes we even get to read a character's extended thoughts, such as when the narrator includes excerpts from Eustace's journal. For example, we learn a great deal about Caspian when we discover that, during the adventure at the Lone Islands, he "was sorry for the others languishing in the hold of Pug's slave ship, but he could not help finding the rest of that day enjoyable" (3.83). Clearly Caspian feels sympathy for his friends, but this sympathy is balanced by his ability to find joy in the moment.
The narrator of The Voyage of the Dawn Treader doesn't like to give us much interpretive leeway and doesn't hesitate to tell us what to think of a character. For example, we are told that Eustace "liked bossing and bullying" (1.3), that Reepicheep is "the most valiant of all the Talking Beasts of Narnia" (1.39), and that Ramandu is "mild and grave" (14.1). Typically, our narrator gives us these direct assessments of characters when they are introduced, and then lets their actions and thoughts characterize them from then on out.