Lucy in the book is blonde. Georgie Henley is brunette but captures Lucy perfectly. The two boys' hair colours are never mentioned but illustrations have Peter brunet and Edmund blond - this is inverted in the film. The only Pevensie to keep the same hair colour is Susan.
Jadis is also commonly depicted with black hair - and she was going to be in this. However they changed it to blonde so as not to make her look too gothic. Tilda Swinton received unanimous critical praise for her performance.
Awesome, Dear Boy: Jadis is one of Tilda Swinton's favourite roles to play and she reprised it in the subsequent films - even when the character didn't actually appear in the booksnote In Prince Caspian some characters talk about resurrecting her; in the film they partially succeed. In Voyage of the Dawn Treader an apparition of her appears to prey on Edmund's fears. She likewise expressed excitement at getting to play her in a potential film of The Magician's Nephew.
Corpsing: The reaction shot of Peter to Aslan's "Beaver mentioned you planned on turning him into a hat" wasn't in the script. A fly was buzzing around William Moseley's head and he couldn't stop grinning.
Creator Cameo: Douglas Gresham (co-producer and stepson of CS Lewis) voices the radio announcer during the children's first night at the professor's.
In the Walden Media film, Tilda Swinton (White Witch) flirted with Skandar Keyes (Edmund) to make him feel more uneasy for their scenes together.
The entire film was shot in near-chronological order - so that the characters' emotional growth would be reflected by the physical growth of the actors. The obvious exception is the scene of all four falling out of the wardrobe at the end.
Georgie Henley was brought onto the set blindfolded when filming Lucy's first arrival in Narnia. As such her reactions to the wood are genuine. Likewise she hadn't seen James McAvoy in his costume before filming the scene, so her reaction to Mr Tumnus is genuine too.
Skandar Keynes likewise wasn't shown the set beforehand, so Edmund's reaction to Narnia is his own as well.
Edmund was the last of the children to be cast, so the other three actors were already familiar with each other by the time Skandar Keynes was brought in. As such this adds to the detachment between him and the others. Also Skandar Keynes hates being hugged, so the director had the others Glomp him at every possible opportunity to drum up the animosity.
Executive Meddling: Lewis was dead when the books were re-ordered. He had no intentions of re-ordering the story himself, his only public expression on the order of the books being a simple letter to a child telling him to read them in whatever order he thought best.
Georgie Henley ad-libbed "My mother's name is Helen" when she meets Mr Tumnus. Notably the Pevensie mother wasn't given a name in the original books - making this a case of Named by the Adaptation (Helen being Georgie Henley's mother's name).