I wrote this in a thread at Facebook, and wanted to copy it here for safekeeping.
A film is not a derivative of the written work. It aught to stand comfortably on its own merits, in every single case. Books are amazing. As are stage plays. But you can do things in each that you can't do in the others. In every case you do your audience a disservice by neglecting the finer points of craft when telling a story in your chosen medium.
If the storyteller wanted me to divine the tone of the written piece, they should have shot and edited those scenes appropriately. In film there IS NO written piece.
So if you're adapting a book to film, get the film right. You simply can not assume that your audience will bring along baggage from the text -- that's lazy storytelling.
This is getting off topic, but I feel the same way about screen adaptations of stage pieces. Amadeus, for example, fully embraces cinematic visual language and is an impeccable film. Even if it came from a play. To pull another example from the other end of the spectrum, Chicago is at best a mediocre film which relies too heavily on the audience being in love with the musical.
The Harry Potter films don't rely on any sort of familiarity with the books (the first film, anyway) -- they're incredible. Twilight (first film) is marginally watchable mush, but if you love the books you're incapable of not buying a ticket. I respect the cash-flow you can generate from that strategy, but I do not respect the work.