Ability over Appearance: Philip Pullman had something of a reaction like this when Nicole Kidman was cast as Mrs. Coulter in The Golden Compass. The character has black hair in the books (Kidman being blonde) and Pullman said "I was wrong, she has to be blonde", Kidman having been his personal choice for the role.
Executive Meddling - The director attempted a more faithful adaptation. The money men differed. See the main page for the result.
Replacing the original voice of Iorek, a Shakespearean actor virtually unknown in Hollywood, with the instantly recognizable voice of Sir Ian McKellen.
Fake Nationality: French (with an English accent) Eva Green as a Scandinavian witch.