It’s an interesting detail about the Narnia books that each one has a (small) group of main characters (typically three or four), allowing “group dynamics” to be one source of tension in the story. Typically, one character in particular behaves badly, making life difficult for the rest of the group (e.g. Edmund in The Lion, The Witch, and the Wardrobe).
While having one character in the group of heroes be a problem is realistic (no group of people always agrees/gets on all the time), it does mean that you tend to get really annoyed at that character. In Prince Caspian, it’s Susan who is the target of shake-some-sense-into-them ire. Through the children’s travels to meet up with Caspian, she complains, belittles, and generally acts like a bit of a brat.
In the film version, however, it’s Peter who acts childish. Now, to some extent I can understand the changes made for the film. Spending 2/3rds of the run-time with the children trekking through thick woods and dealing with getting lost wouldn’t work on-screen, so it has to be cut. This means Susan has nothing to do in the film, so concoct a flirtation between her and Caspian (a bit weird, but in keeping with Susan’s character development over the series). Moments that are described briefly in the book have to be fleshed out, so we spend more time on Nikabrik’s plan to resurrect the White Witch, and on Peter and Miraz’s duel.
What I don’t like, however, is the way they change both Caspian and Peter. In the book, both behave maturely and show a great deal of respect for each other. But of course, that’s boring, so in the film: macho posturing. *sigh*. They even invented a failed attack on Miraz’s castle to explain the sorry state of Caspian’s army, which they wouldn’t have had to do if they’d had him blow the horn when in actual need as opposed to having fallen off his horse and gotten a boo-boo.
Excuse my ranting, but being reminded of what the original story is, it’s annoying to see it depicted in quite a different manner, and with quite a different theme/message. The film seems to follow the classic “pride goeth before a fall” character arc (hero thinks they are great, hero’s big plan fails miserably, hero introspects <- emotionally powerful scene!!!1!!, hero gains humility and eventually succeeds). The book focuses far more on the idea of having faith in something/someone, even though you can't see how things will work out – a less common theme, that could have given the film something different. Unfortunately, something different is anathema to risk-averse hollywood studio-types (which is a whole other topic).