The fall of Lucy
I know this is under dispute
elsewhere, but I think the film did a magnificent job of portraying the Fall of Lucy with the spell of Beauty Beyond the Lot of Mortals. This was difficult, because in the book this is an extremely significant incident, the importance of which escapes most readers. If you read the account carefully and ponder its implications, you realize that
Lucy was willing to lay waste the Narnian world by saying the spell. From the pictures on the page, she knew the devastation that would result if she said the spell, but she wilfully set her teeth and was prepared to say it anyway. Only the direct and miraculous intervention of Aslan prevented her - and even then her petulance about not being allowed to say it resulted in her saying the Eavesdropping Spell.
The significance of this incident is hard to overstate. Every human who comes to Narnia falls at some point (with the exceptions of Peter and Polly Plummer - and Polly has a bit part.) Edmund's treason, Eustace's petulant selfishness, Jill's pride at the cliff edge, Digory's willfullness in the Hall of Images at Charn - everybody falls. Lucy's fall came here, standing at the reading podium in Coriakin's house. Lewis clearly implies that it is her envy over Susan's beauty which causes her rebellion. There was probably more involved - like most young girls Lucy probably admired many beautiful women - but as her sister, Susan would have been a focal point of Lucy's dissatisfaction.
The screenwriters had a task with this one. Books have a narrative voice, so Lewis could describe what Lucy was seeing in the pictures on the page, allowing the reader to understand the situation. Films have no such device. They must rely on dialogue and images, and have very tight time constraints. I think that an attempt to directly translate what Lucy saw on the page in the book - i.e. the devastation of the Narnian world - would have been confusing and distracting. Also, the appearance of Aslan's face on the page would have mystified people, and the message would have been lost.
I think the modifications made by the screenwriters beautifully captured the heart of Lucy's rebellion without violating the core of the story. Her tearing out the page and tucking it under her tunic, then getting it out and reading it later, told the tale very effectively. The scene in the mirror where she is rewarded by "becoming" Susan, followed by the nightmare of realizing that she had "wished herself away", was very powerful. Lucy found that (to use Aslan's words), "all get what they wish, but they do not always like it."
I think the film made good use of the screen's ability to leverage ambiguity. There is no question that Lucy said the spell, and consequences followed. Aslan said as much, confronting her frankly with her deed ("What have you done, child?") Lucy must acknowledge and repent of what she has done. Then she "wakes up", and we're left unclear about whether the whole thing had been a nightmare, or if Aslan had somehow undone the consequences of Lucy's rebellion.
The critique has been made that Aslan's rebuke of Lucy was couched in the pop-psych jargon of self-esteem, and I think there's validity to this criticism. Yes, the gravity of Lucy's rebellion was much more severe when it was clear that people would die and nations be destroyed as a consequence of her disobedience. But given the limits of film, and how the screenwriters recast the incident, I think Aslan's response was the best that could be done. After all, the "wishing oneself away" is not just a personal unpleasantness, but a grave sin. All in all, I think the film accurately and potently portrayed the fact that Lucy did wrong, knew she was doing wrong, and was reprimanded by Aslan for her disobedience.
One thing I wish could happen: on the page in the book that "showed" Lucy with Susan's face, if the titles "Vogue" or "Cosmopolitan" or "marie claire" could be superimposed on the mirror, that would really drive the message home. Despite the blather about valuing yourself and having high self-esteem, the real message sent to young women in our culture by such media is that they are not satisfactory, that they should envy the beauty and glamour of others, and they should try to become someone they're not. They are told to "wish themselves away".