It's pretty ironic that they tried to tie in VDT with snow to remind the audience of LWW and now they actually have a movie that can do that with and keep it canon and relevant.
I would say that the most important part of any book is theme first, plot second. The changes to the plot in An Unexpected Journey would not bother me if I hadn't felt that the themes were totally changed and that made me like the movie significantly less. This applies to any book to movie adaptation. The themes of faith and trust should come through and then the plot changes will matter less. We may still not like them, but readers of SC should leave the movie theater with the same feeling they get from reading the book.
I also agree that Puddleglum's speech should be kept verbatim. From what I've seen on the forum, the lines from his speech are included in pretty much everyone's favorite quotes. They could be seen as the epitome of faith in the book and are thus necessary for the message and plot. If they water it down, fans will probably be extremely disappointed.
My last bit of advice that I think all NWers will agree on: get people who care about the book. VDT made me lose all hope in book to movie adaptations, so gain my trust back. (Ironic, considering the theme of the book)

My message to the filmmakers:

Keep it simple.

The Silver Chair doesn't have to be epic, it just has to be good storytelling.

Just pretend you have a budget of $50 million. What would the script look like? How would you keep the audience's interest? -By creating a character-based story that is interesting to watch; not because of the action scenes or the massive special effects, but because of the choices the characters make. We want to see what our heroes will do next, and so we keep watching. All the clues that you need to make a compelling drama are right there in the book. Just be sure you are working with the C. S. Lewis Estate to ensure you are interpreting the characters correctly.

If you happen to be blessed with a budget higher than $50 million, then great! Use the extra money to pepper the movie with breathtaking scenery & jaw-dropping special effects. But always remember that the effects must serve the story, and the story is about character, not battles. Special effects are the spice of a movie, not the main course. Nobody likes to eat a bowl of pepper.

Don't m**f the signs. The signs are supposed to be stepping stones to the finale of the movie NOT the plot. Push the drama in the story around the signs if you must but don't mess with how the signs are ignored or muffed by the characters.

My one request is this: Don't lose the magic and joy of Narnia. LWW came closest to capturing this, but it was lost in PC and VDT.

I know many kids who go looking through closests and cupboards trying to find their way into Narnia after reading the books. But the movies removed most of the reasons I ever wanted to go visit, and they were replaced with action and conflict. Not a happy and magical place. I can read the local news if I want action and conflict.

Adding joy to the SC is a bit of a trick, I admit, as there's not much to be had. But there are two important scenes I can think of to counteract the darkness of the rest of the story. Golg's excited description of the incredible underworld with edible gems and liquid gold is one of them. Rilian and Eustace were ready to go visit! And the second is the snowball dance the party falls into when they finally escapes the underworld.

Please don't lose this. It's SO important to Narnia.

Be creative.

I want to see something I've never seen before, I want to see something I haven't imagined or anticipated. Don't try to please the fans, don't try to make everything "family friendly", don't try to appeal to any specific audience for that matter. Rely on your creativity as you write, direct, cast, design, and perform the film. Producers: don't throw together the components of the film as if you were making a recipe; hire directors and writers who have a passion and vision for the film, and allow them to use their creative talents.

I would much rather spend my $10 on a creative work of art that is flawed, and divides and frustrates (Prince Caspian), than a bland and tasteless hack job trying to profit from a brand (VOTDT).

I couldn't care less whether your work exactly matches my original interpretation of the book, because I already own the book and have read it like 7 times: I want to see another side to the story, I want to look at the story in a whole new light. But at the same time, please don't insult me and that interpretation by expecting me to swallow the one you've allowed to be corrupted by too much studio influence or fan-service.

I cant stress this enough. Don’t go over the top with Puddleglum — he isn’t a clown, and is not in the story purely for comic relief.

I would agree with many here on paying attention to the source material for the directors. I think also a sweet Narnia feel should be to it, and Christian themes should be clearly outlined in it. Dawn Treader had some of these things, but it also strayed from the book.

Dialogue. Good dialogue is very difficult to write and, in my opinion, one of the few common weaknesses of all three films, though off the top of my head I think LWW was the best. The screenwriter's attempt to modernize the characters' speech led to some dreadfully insipid dialogue. Many kids are smarter than we give them credit for and the characters would deepen and just plain sound better if they left the dialogue closer to how it appears in the book. In fact, when in doubt, copy and paste . I certainly realize that wouldn't actually work for a film, but if they started from that, and made changes and edits when needed I think it would settle itself and actually take a lot less work than essentially translating it into language the screenwriters believe is more accessible.

Many people don't realize the importance of how things are said. If a character says something that is from the books, but says it in a fashion that he/she would never do, that is infinitely more of a deviation from the spirit of a novel than having the character say something that doesn't appear in the books, yet they say it in a fashion that is truly how the character would. An example of this is Eustace. Sure, very little of his lines were actually from the book. Yet the reasons I overall approved of the character in the movie is, one; Will Poulter acted the character well, and two; many (though certainly not all) of his lines were true to the Eustace character. I should clarify that I am not talking about mannerisms, voice, or anything actor-driven. I mean simply the arrangement of words. Jill conveys the same information as Eustace with a different arrangement of words. Puddleglum certainly sounds unlike anyone else. A novelist uses words like a musician uses notes. Two songs may be made up of the same notes yet sound entirely different. In the same way Lewis has his characters speak in different melodies to help them sound distinct without being actor-driven, because of course it's the written word.

Oops. Went on a bit there. In summation, use more quotes from the book .

Also, the tone of this film should be fairly dark. I know many people accuse Prince Caspian (film) of being dark, and they mean that as a negative, but The Silver Chair is dark. No way around it. It's bleak, grey, cold, wild, raw, and literally dark. Very little of the book takes place in Narnia(by the way, spoilers if anyone cares), and those scenes should capture the light that they discover is under threat. But after that we have marshes, Ettinsmoor, the wild wastelands of the north, a screaming blizzard (which should not be thematically connected to LWW as one scene is full of wonder and discovery and the other misery, suffering, and dissent), and then an extended period under ground with cold, weak lighting. They end up fleeing from the only light and warmth in the book (the fires of Harfang, and the Lady's fire).

Even content-wise there is a certain heaviness. The characters spend a lot of time angry and bickering. The death of the queen, death of Caspian, the eating of the talking stag, the giant's recipe book and the realizations about the feast, the beheading of the serpent. This is all pretty intense stuff. I am not saying they should go for a higher rating. I don't care about ratings, that's all marketing stuff. Aiming for a certain rating is letting marketability dictate the work of art. I think they should make the best adaptation they can and let the ratings fall where they may. But I know ratings always matter. I just don't want them to sacrifice meaning and art to appeal to the "family movie" market.

I hope no one misunderstands me. I'm terribly excited and all this comes from enthusiasm for the possibilities. If anyone has read this far thanks for slogging through, and sorry for going on so long. I'll to keep things succinct in the future.