Narnia is one of the rare occassions where the movie takes advanatge of being a different medium to enhance the book considerably. This enahncement worked on multiple levels and was clearly done by a director and screenplay writers familiar with original and determined to stay close to the intent of the author. Notably, unlike Peter Jackson's modernist retelling of the Lord of the Rings, Narnia remains emphatic about the glory of war for good and the need for good to have the same terrible and awful powers as evil. (Read Lewis' introduction to The Screwtap Letters for his flame against those who would portray good as all soft and cuddly and shy away from good as terrible and frightening in its own way.)
On a simple level, the movie is gorgeously rendered, and the use of modern anamtronics makes it a much more "real" experience than animation. The movie manages the rare feat these days of balancing drama and humor. Even better, the humor does not feel forced or out of place. The by-play between the beaver married couple, for example, feels like an English country family from the 1940s. They bicker but also clearly express affection.
Visually, the filmakers take advanatge of the medium effectively. For example, from the time the four siblings enter Narnia until the time Edmund makes his break for the White Queen, Edmund is visually just a bit apart from his three siblings. When they stand at the lampost, when the enter Thomas' home, when they sit at the beavers' it is as a group of three and one, rather than a group of four. After Edmund's return and redemption, this separation vanishes.
Similarly, there is a clear visual similarity between the blitz and the griffin "air force" deployed by Peter in the battle for Narnia. In the hands of Jackson or Spielberg, this would be used as a condemnation. Under the doctrine of modern equivalents, where force is bad and heroes must reluctantly only go to war after all else has failed, this would be used to suggest that Peter has descended to the same level as the Nazis. But Adamson has created no such doubt. Consistent with the intent of the original and all else in the book, the symbolism suggests that the difference between good and evil is not means, but intent. Peter has taken the strategems of evil and employed them against the forces of darkness, rather than contaminated the forces of light by adopting the methods of evil.
On the storytelling level, the inclusion of the blitz and the extended interaction between the children before entering Narnia provides both context for Edmund's "fall" and a new storyline in Peter's maturation.
To proceed somewhat more linearly. We open with the blitz. This sets the chemistry dynamic of what is to follow. Edmund is clearly grieving for the absence of his father. He is drawn to the incoming planes as a moth to flame. Not merely because he is, after all, a young boy, but because his father has gone to fight these same terrible machines. Edmund rushes back to the house to retrieve the picture of his father -- his one physical link. Peter restrains him, calling him disobedient and selfish. This sets the stage for their conflict. For Edmund's act is not selfish in the classic sense of being for personal gain. It is an act of sentiment and emotionalism. OTOH, it is selfish in that it puts his personal wants and desires above all else as the most important thing in the universe. Edmund resents Peter as asserting an authority that, rightfully, belongs to their father. To him, Peter is a usurping tyrant, belittling his feelings and ignoring his needs while showing sympathy and generosity upon the others.
It is interesting to contast the treatment of Edmund here with that of Lucy. Lucy also disobeys the evacation of the house initially, hiding in her bed. But she is more obviously a sympathetic figure of a frightened little girl and baby of the family. As a result, she is treated by Peter and the rest more sympathetically than Edmund, further driving the wedge between Edmund and his family.
But Peter is also laboring under a burden of his own. He is placed in a position of responsibility well ahead of his years and not of his asking. Edmund's attitude toward him sparks a natural resentment. After all, Peter did not ask for this job, and Edmund is only making it harder. The stage is thus set for Edmund's fall from grace and seduction by the White Queen.
On arrival, the children are placed in an unfamiliar and stressful situation. They have been transported from their home to a museum, seperated from their family, and expected to be grateful for the privilege. Adamson develops this modern tension which Lewis glosses over in the book, making the characters, and their evolution, more real. Within the PRofessor's house, the personalities of the children continue to play out and symbolism becomes more established. Lucy demonstrates innocence and acceptance, Edmund resentment and loss, Susan represents intellect, Peter responsibility.
But Peter's sense of responsibility is still blinded by his resentment of Edmund, as displayed in the subtle ways he treats everything as Edmund's fault. The Professors' injunction that he and Susan should start "acting like a family" applies to more than just believing Lucy about the wardrobe. Peter's failure in this regard is summed up in his reaction when Edmund breaks the window - clearly an accident incidental to the game. His puting the blame on Edmund is a classic older sibling/sibling rivalry rather than the responsibility of a father figure. This enhances Peter's subsequent evolution into a true leader. He achieves his leadership not merely by right of being eldest, but by graduating by turns into a leader capable of looking past his personal feelings to the broader sense of responsibility. His realization of his responsibility for Edmund's alienation ("we were too hard on him") and subsequent acceptance of Peter on his return without reproach (accepting Aslan's statement that "there is no need to discuss the matter further") mark personal growth and growth of all of them in acting like a family.
Similarly, Peter evolves through not wanting any part in the conflict, to wanting to send everyone else back while he fights the war (paralleling his father) to accepting that his responsibility to keep his family safe yields to his higher responsibility to Narnia and, ultimately to his family and their destiny. He matures and realizes he serves them better not by shileding them, but by alowing them to undertake the dangerous work of saving Narnia.
Moving on to Edmund's seduction by the White Queen and subsequent betrayals. Adamson treats Edmund with considerably sympathy. In the pure christological interpretation of the book, Edmund's fall is the fall of Adam, not the betrayal of Judas. This is why Aslan's sacrifice redeems Edmund (whereas in the Gospels, Judas Escariot commits suicide and is condemned to Hell).
Edmund is seduced by the White Queen's apparent show of compasion. Also, it is not as if he has reason to suspect the White Queen. Lucy has not, at that point, warned him about her. True, the observant might be suspicious of the callous way in which the Queen treats her servants. But Edmund is seduced by her attentions and flattery. His subsequent betrayals are motivated b things beyond pure selfishness. For example, his coming to the White Queen is motivated in part by affection as well as greed. His confession of the return of Aslan is motivated by his fear and also because he does not realize how serious a betrayal it is (when he sees the look on Thomas' face, he stops short of telling her Aslan's location). His final betrayal is to save the life of another.
Like Peter, Edmund also grows. His final defiance of his brother's orders in the battle of Narnia, mirroring his decision to run back into the house during the blitz, is an act to save a needed authority figure at the risk of his own life. But his actions are no longer selfish even to the extent of setting his own needs ahead of others. To the contrary, he demonstrates the capacity for sacrifice for the greater good.
The evolution of Lucy and Susan is sadly less developed, but still works. Lucy, as innocence, learns to fight and take action into her own hand. Susan, as intellect, learns the importance of responsibility and self-sacrifice. It is she who is, initially, the one who suggests to Peter that it is the rational thing to do to accept the White Queen's offer of Edmund in exchange for their departure. Ultimately, however, she understands that because they are the people who can make a difference, they have a responsibility to do so, even if the rational thing to do is simply to return to the wardrobe.
In any event, I'm looking forward to getting it on DVD when it comes out. I highly recommend it as a movie that is an excellent work of craftsmanship, not merely a movie for fans of the original who will be pleased to see the story preserved.