The original is actually a relatively thin book, only 99 pages. Think about it, the later Harry Potter books have six or seven hundred pages.
Let me talk about the difference between the movie and the original.
1. The plot of the siege in the movie is not in the original book. When several children met the prince, the prince was being seduced by three of them: werewolves, dwarves and witches. The conflict between Prince Caspian and Peter seems to be in line with their ages, but Peter in the original book is much more mature in this regard, because he was an adult in Narnia before. The first thing he did when he met Prince Caspian was a solo challenge to Miraz, apparently having a lot of experience with it from his previous days in Narnia.
2. The love affair between Prince Caspian and Susan, who is talked about with great interest, is also absent in the original book. Susan and the others came to Aslan's How after crossing the river, only 2 boys went in, the girls stayed with Aslan all the time. Prince Caspian and Susan basically had no chance to meet.
3. Prince Caspian's teacher escaped after the prince escaped. The prince knew from a young age that he was half man, half dwarf. And he can do some magic.
4. There are not many descriptions of Prince Caspian in the original works. Much of it was spoken through the mouths of dwarves.
5. The reason why the Lord killed Miraz was because Miraz called him a coward. By the time the Telmarine soldiers reached the river, the bridge was gone.
Mainly just a few points.
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