#forever screaming about this
I genuinely think the most of the differences between the Pevensies in the books vs. the movies can be explained by one simple fact: the Movie!Pevensies are fully cognizant of and emotionally attached to their experiences in Narnia after returning to England, while the Book!Pevensies have a notable emotional distance to their memories of Narnia
It’s an explicit plot point in the books that the longer you stay in one world, the easier it becomes to forget about the other one (or think of it as a dream). Additionally, while the Pevensies do remember what happened and are permanently changed by their experiences in Narnia, they seemingly forget a lot of their Narnian-learnt knowledge and skills. This gets briefly mentioned in Prince Caspian, when Lucy talks about swimming:
When they had drunk from the well and splashed their faces, they all went down the stream again to the shore and stared at the channel which divided them from the mainland. “We’ll have to swim,” said Edmund.
“It would be all right for Su,” said Peter (Susan had won prizes for swimming at school). “But I don’t know about the rest of us.” By “the rest of us” he really meant Edmund, who couldn’t yet do two lengths at the school baths, and Lucy, who could hardly swim at all.
“Anyway,” said Susan, “there may be currents. Father says it’s never wise to bathe in a place you don’t know.”
“But, Peter,” said Lucy, “look here. I know I can’t swim for nuts at home—in England, I mean. But couldn’t we all swim long ago—if it was long ago—when we were Kings and Queens in Narnia? We could ride then too, and do all sorts of things. Don’t you think—”
“Ah, but we were sort of grown-up then,” said Peter. “We reigned for years and years and learned to do things. Aren’t we just back at our proper ages again now?” -PC, Chapter 3
So she learned how to swim while growing up in Narnia, but forgot it again when she returned to her own world and was back at her proper age. But the longer the Pevensies stayed in Narnia, the more those previously-learned skills came back to them:
This was real broad-sword fighting. The great thing is to slash at your enemy’s legs and feet because they are the part that have no armor. And when he slashes at yours you jump with both feet off the ground so that his blow goes under them. This gave the Dwarf an advantage because Edmund, being much taller, had to be always stooping.
I don’t think Edmund would have had a chance if he had fought Trumpkin twenty-four hours earlier. But the air of Narnia had been working upon him ever since they arrived on the island, and all his old battles came back to him, and his arms and fingers remembered their old skill. He was King Edmund once more. -PC, Chapter 8
The same general principle appears to apply to their memories and the emotions attached to them (which I think is one of the main reasons why Book!Susan is so easily able to dismiss Narnia as a “silly children’s game” when she gets older). Narnia’s magic protects visitors from feeling the pain and suffering associated with being de-aged/losing their home/etc when they leave Narnia by forcing a mental and emotional distance from their experiences, and they’re only able to re-forge those connections once they return.
By contrast, the Movie!Pevensies are acutely and painfully aware of what they’ve lost by returning to England, and it dramatically alters the equation for everyone involved.
It turns the level-headed, loving, tired Books!Peter into the angst-ridden, hotheaded Movie!Peter who’s constantly trying to prove himself. It turns Susan’s story inside out, because it gives her entirely different reasons and motivations for possibly wanting to forget about Narnia and dismiss it as a “silly game.” It gives Edmund a narrative reason to retain the massive character growth he’d experienced in LWW, act as the steadying rock in the siblings’ lives, and feel like he has something to prove in VDT without Peter around.
And while I think Lucy remains the least changed between the books and movies, there are several added dimensions to her fully remembering her first fifteen years in Narnia that really start to peek their heads out by the time we get to VDT (particularly her desire to feel grown up and desired by the men around her; if she’s truly feeling her age, I think there’s an additional nuance to that plot that goes beyond the vanity and “she wants to be like Susan” dimensions from the book).
Do I think this accounts for all of the characterization differences? No. But I think it’s a massively understated difference that doesn’t get talked about enough as being a major driver of how differently the Pevensies act and react to things in the later books vs. how they act and react in the movies.
