My mother requested a blog post about the differences between the recent movie of "Voyage of the Dawn Treader" and the original book by C.S. Lewis. I am simultaneously well-equipped to write about this, because Dawn Treader was my favorite of the Chronicles of Narnia, and ill-equipped, because it has been years since I read it... I might have to go read it now, actually... But I'll comment now on the biggest differences I noticed (and fact-check after I reread the book).
First, let me note that the reason I loved Dawn Treader the best is that it is one of those voyage/adventure books that feels like it could go on forever. The general structure is of traveling from island to island on the ship the Dawn Treader, sailing the uncharted seas on the outskirts of Narnia. In my recollection of the book, the various island episodes are fairly unconnected. In fact, in the book-on-tape version that I listened to (over and over) as a child, several of the middle islands were simply omitted, to be replaced by the line "And they had many more adventures." So when I eventually read the book for myself and discovered the Dufflepuds and other characters, I was thrilled to find more new story hidden inside this familiar book. This is perhaps the reason that I feel the book could just keep expanding, and why it is so clear to me that the island episodes in the book are relatively independent.
In the movie, however, this is not the case. To closely connect the islands and, I suppose, create a faster pace and more suspence (Heaven forbid there be a movie that actually takes its time in this day and age!), the movie added an overt villain: the Scary, Creepy Mist. At the first set of islands that Caspian and his crew visit, they find that the Mist has been menacing and kidnapping the Narnian inhabitants of these isles. Moreover, the Mist is getting increasingly powerful. The message of "Soon, it may be too late" hangs in the air. This discovery increases the urgency of their quest to find the Seven Lords, because the way to defeat the Mist (rather bizarrely, to my mind) is to lay their Seven Swords on Aslan's Table. They learn this and are commissioned by the sorcerer at the second island that they visit.
So at every island the Dawn Treader visits, and even while the ship is at sea, the Mist keeps tempting its crew. Lucy, Edmund, Caspian and especially Eustace Clarence Scrub are each tested (I won't say who passes and fails). Lucy's test has to do with envy; Edmund and Caspian are tempted by greed and status; and Eustace must outgrow his selfishness. Meanwhile, the themes of courage and self-acceptance keep surfacing. "Love yourself as you are" and "Have courage, take risks for the greater good" come across as the lessons of the movie.
Courage and self-acceptance are both good messages, but they are not the messages of the Dawn Treader that I remember. In refocusing on those two ideas, the movie reshapes many aspects of the book. The order of the islands is changed, two islands are combined into one; the Mist becomes the enemy to defeat; the voyage of the Dawn Treader becomes a quest to save the world, in which the actions of individuals hold great weight; and the power of Aslan is diluted.
That last assertion might seem like a bit of a leap. Let me explain.
Both the book and the movie explore the idea of transformation. The most powerful instance of this is Eustace's transformation, as physically enacted in his turning into a dragon and eventually regaining his humanity. The book and movie treat this episode very differently. In the book, as long as Eustace is a dragon, the whole crew is stuck at the island where he was transformed. He helps them repair their damaged ship during the day, and at night he mourns his plight. After many days (I believe), he is somehow led to a pool, and has an encounter with Aslan. Eustace scratches off his dragon skin, and wriggles out of it. He feels lighter, freer. But when he looks in the pool by the light of the moon, he finds he looks just the same. He is still a hideous dragon. So he peels off another layer. And another, and another. I can't remember how many he has to remove, but each time it feels less pleasant, though he can see himself shrinking and shriveling, getting closer to boy-size, leaving dragon-dom behind.
But at the last, he is still wearing a dragon-skin and he can't bring himself to score it deeply with his claws so that he can finally writhe his way out. In the end, he needs Aslan to lift his heavy paw, and slash this deeply hidden dragon-skin wide open. It hurts, it hurts, but when the skin peels back, finally, Eustace emerges as a human again. Victory has come to him, and victory has made him small and weak and naked. When he returns to life as a human, he lives differently. He has been humbled, and he has had to see that he needs Aslan.
So the lesson of the book's dragon-transformation is surrender and dependence and reliance on Aslan (God). Transformation takes time and pain and the deliverance of a higher power.
But in the movie, Eustace is redeemed from dragonhood only after he saves the ship and its crew from the Mist, by fighting the seamonster/dragon that the Mist makes itself into. Eustace has to demonstrate that he has learned courage and confidence (largely from Reepicheep) before he can be returned to his true form. When he is changed back into his boy's body, the transformation does happen through Aslan's intervention, but the process is full of flashing light, fire, red and gold, and triumphant music, and brilliant daylight. It's not darkness and a hidden moonlit pool, it's not humility and pain and labor. Instead, it's magic. Aslan doesn't even actually touch Eustace; he just makes scratching marks on the sand and suddenly swirls of gold and bright sparks surround the dragon-Eustace and lift him up and spin him around. When the cloud of glory disperses, Eustace has become a boy again. Abracadabra!
In the movie, Eustace earns his humanity by fighting the dragon. Then, like clockwork, Aslan appears in the middle of the sunlit sandbar, draws runes on the sand and bam! Eustace is a boy again. Here, transformation or redemption is not initiated by Aslan/God, but rather by the good deeds of the one to be transformed. Moreover, the process is fast and sparkly, instead of slow and almost tortuous, as though it's just a superficial change in appearance, rather than a deep inward change created by Aslan's redeeming work.
This episode captures the fundamental difference between the book and the movie. In the book, everything depends on Aslan. There will be no fatal consequences if the humans fail in their mission to find the Seven Lords. Aslan is what really matters. In fact, I believe that in the book, a major reason for their quest is the search for Aslan's country. This does not feature at all in the movie as a motivation.
In the movie, you see, human effort is vital and effective, and transformation comes through willpower and courage, and you earn it, and ultimately it depends on you. Saving Narnia from the evil Mist also depends on you. If you fail, it could be the end of everything. Aslan has his table and his country, but he's not doing anything to help you out along the way. You are on your own.
Ok, that last might be an overstatement, but it's not far off. Let me just put this really succinctly: the book is about grace, the movie is about effort. That is the difference, and that is why Voyage of the Dawn Treader is one of my favorite books, but "Voyage of the Dawn Treader" is not one of my favorite movies.
p.s. Please comment and correct me if I got facts wrong!