I think there is grave danger if a movie tries to be TOO loyal to the book. They are, after all, different media entirely.
Our characters are about to enter a forest. A book can devote several pages to description of the forest. In the movie, you see the entire forest in seconds, and it can’t take up too much screen time. This is a strength of the visual medium over the verbal medium.
A book can describe a character’s inner thoughts. A movie must show those thoughts visually, or use some lame device (like the actor’s voice as “thoughts”). This is a strength of the verbal medium over the visual medium.
A book may take six or seven hours (or longer) to read. A movie that runs more than about two hours will have most of the audience looking at their watches, shuffling in their seats, and going out to the bathroom. The only way to film an entire book is to do it as a miniseries, and that loses the big bucks and the big screen.
Thus, the filmmakers of LORD OF THE RINGS have every right to, and MUST, make changes. So far, all the stuff I’ve seen LOOKS great: characters, settings, visual effects (I love the shot where the writing on the ring is reflected on Frodo’s face!.. the book doesn’t do that, of course, but that’s using the filmic techniques to convey emotion/situation that the book conveys more directly.)
If they need to drop secondary characters (like Glorfindel) and give the role to another secondary character (like Arwen), well, that’s fine by me. If they wind up stressing Eowyn and downplaying Eomer, that’s fine too. Tolkien was writing great mythic adventures in an era before our current approach to gender equality. If he were writing today, how would he have done it different? That doesn’t bother me.
My only question is whether the movie is good-storytelling, based on the book.
It seems to me silly to prejudge based on the trailers. Trailers are designed to attract attention, and a femal elf warrior attracts the attention of the Xena-set. Fair enough. It seems also silly, IMHO, to prejudge the choice of actor/actress: “OHMIGOD, they’re using a has-been vaudeville actor to play the Scarecrow! That movie will be a disaster!”
If you want an example of a movie that was very faithful to the book and was therefore borrrrrrrrring as a movie, I point you to HARRY POTTER. I can’t imagine going back to see it again. The movie was absolutely faithful to the plot of the book, but lost the heart, the spirit… at least, IMHO.
No, let them take liberties to make a good movie.