Character Analysis
Eustace Scrubb doesn't get a whole lot of character development in The Silver Chair. That's because Lewis was relying on all the work he put into creating (and reforming) Eustace's character in The Voyage of the Dawn Treader. Listen to the first, unpromising description of Eustace from that book:
There was a boy called Eustace Clarence Scrubb, and he almost deserved it… he was quite glad when he heard that Edmund and Lucy [his cousins] were coming to stay. For deep down inside him he liked bossing and bullying; and, though he was a puny little person who couldn't have stood up to Lucy, let alone Edmund, in a fight, he knew that there are dozens of ways to give people a hard time if you are in your only home and they are only visitors. (VDT 1.2)
Eustace has a serious change of heart on that voyage—after experiencing fierce seasickness, the hatred of most crewmembers, a full-on transformation into a dragon, and a vicious de-scaling by Aslan's claws, that is. His time in Narnia creates a total change in him so that by the opening of The Silver Chair we have a boy who catches a friend in the middle of a cry and responds like a human being to her.
Part of his new persona includes a sense of humility and the ability to feel sorry for his bad behavior. When Jill tells him that she's noticed his improvement, he's pleased, even though he still feels chagrin over his past actions: "… gosh! What a little tick I was" (1.5), he says. But it's his willingness to put it all on the line and reach out to Jill—in a way that could be really embarrassing at a place like Experiment House—that brings them into Narnia.
Imperfect Understanding
Although he's had close contact with Aslan, Eustace still doesn't perfectly understand the Lion's nature or how connected he might be to Eustace's own world. He doesn't know how he can contact Aslan from the grounds at Experiment House and feels faintly ridiculous in trying.
It turns out that his knowledge of all things Narnian is, in fact, quite limited. On his return to Narnia for his adventures with Jill, Eustace doesn't recognize much about the place (it is, in fact, a part of Narnia that he's never seen). And he knows even less about his new mission than Jill, who is a newbie.
Perhaps Eustace's lack of knowledge highlights that he still has work to do if he wants to become a true Narnian. As he moves through the adventure, however, he assumes the strength, courage, and desire for adventure that he gained on his previous mission with his cousins and Caspian.
This is also something that vexes Jill: "She did not realize that he had been on great adventures in that world before and that the Narnian air was bringing back to him a strength he had won when he sailed the Eastern Seas with King Caspian" (5.64). It feels unfair to Jill, who has to work very hard to get the hang of things—and who seems to be failing miserably.
But Eustace knows firsthand that failure and painful introspection are the first parts of becoming one of Aslan's loyal and useful subjects. It is in the struggle that such strength and clear-headedness is won.
Eustace and the King
Eustace counts King Caspian as his personal friend, so seeing him debilitated by age sets the young adventurer back a bit. He feels the unfairness of the situation (he's missed the youth of a valuable friend), though he doesn't much complain about it. Facing up to Caspian's mortality gives Eustace the chance to pause and assess the other things in his life that might be greater than his fear of age and death.
In this, he really does resemble his companions, the Narnians Puddleglum and Rilian. As he fights against the wicked queen's enchantments, he searches his memory for the fragments of Aslan and all that he values about Narnia to help him defeat her. And when it comes down to blows, Eustace takes his place next to Rilian and Puddleglum. Though his sword doesn't do much good, Eustace proves that he is ready and willing to lay down his life for Aslan and Caspian.
He's probably not considering it in the moment, but by restoring Rilian to his rightful crown, Eustace is ensuring a kind of immortality for his beloved and dying friend, Caspian.
Meanwhile, Back on the Mountain...
Eustace has learned to follow the commands of Aslan and to have faith in the Lion's plans, because regardless of any immediate personal danger, it is always for the best. As Rilian says to his companions before making a desperate attack into Underland, "Aslan will be our good lord, whether we live or die" (13.191). So when Aslan asks him to pluck a thorn and drive it into his paw, Eustace does it—and very nearly without hesitation. The results are good.
But the resurrection of Caspian has amped up the challenge for Eustace. While he's glad that his friend has been restored to life and youth, he's also a little freaked out—his understanding doesn't reach that far. Although his first impulse is to embrace his friend, Eustace has to get some things straight in his mind. His concerns about mortality, present from the first moment he saw that his friend had aged, persist to the end of the adventure.
Caspian's return to life, then, is a kind of trial by fire to see if Eustace really believes in Aslan's goodness and sense of justice. But although Aslan is asking a lot of this young boy, he's also patient and tries to help Eustace wrap his mind around an important truth: "[Caspian] has died. Most people have, you know. Even I have. There are very few who haven't" (16.240). Say what?
Aslan wants to continue Eustace's moral education and teach him what it means to truly love someone, in spite of death. He needs Eustace to understand that death is not the end of the adventure and that the comfort of friendship transcends such bounds. It's a hard lesson for Eustace to accept (seems reasonable), but Aslan clearly knows that he's ready for it. And Eustace will have to accept it, too, before he moves on to his next great adventure with Jill.
Eustace Scrubb's Timeline