I think it's a combination of a few things including just how faithful the makers are to their source material, although there are ways of being faithful that still allow for changes. Also, though, in many ways, the opinion of whether or not the directors/producers are being faithful to the source depends on how familiar the viewer is with that material.
Take Warhorse for an example. I've only read the book once. In my mind, the movie was pretty faithful to the book and they did a spectacular job. Yes, they did make some changes, but I felt that they kept the characters the same and touched on most of the best moments. But, I didn't (and still don't) know every word of the source material.
The movie version of Snow Flower And The Secret Fan was another adaptation that I really enjoyed. In some ways, I wasn't expecting to either because they decided to make quite a few additions to the book in the movie. However, it was done in a tasteful way, and although the movie-makers added their own characters in the modern story-line, the material from the book itself seemed to me to have stayed quite true to the characters and story Lisa See wrote.
In the same vein, but over to TV, are the two shows that I'm watching this year: Outlander (currently on it's mid-season break) and now the mini-series The Book of Negroes.
So far, the first half of the season of Outlander has stayed true to the book, at least as far as I can remember (it's been more than a few years since I've read Diana Gabaldon's books). Again though, barring a couple of additions, the characters are feeling 'right' to me - and even the small additions work. Still, as I've noted with other adaptations I'm watching or have watched, I don't know every little word or detail of these books.
And with The Book of Negroes, I haven't read the book at all. Which is why I'm asking what you think and how the show compares with the book. I have to say, coming at the show cold, without any knowledge from Lawrence Hill's book, it's been pretty good so far.
Now we come to the big ones when it comes to movie adaptations of books. The Lord of the Rings movies and The Hobbit movies. Everyone around me knows my opinions on the Lord of the Rings movies - although I'm finding a new appreciation of the soundtracks of late. Too many changes - most of which were unneeded at least in my opinion. I also didn't feel like any of the characters were true to their source characters at all. I remember making comments along the lines of "the only thing these characters have in common with Tolkien's is their names". It may simply be that this is the one case where I know too much about the books - I've re-read it more times than I can count by now.
Most of the changes that really bugged me were in two general categories:
- changes to the characters and their motivations. Most of these changes were to the detriment of the characters. Denethor, for example. Where was his dignity? Aragorn too was made less. Sam would never have turned back, even for a moment the way I remember him doing in the movie.
- Unneeded additions. Did Frodo and Sam really need to be at the battle in Osgiliath? Did we really need to see Aragorn falling off that cliff?
Paradoxically, The Hobbit movies felt truer to the source material - barring, of course the entire stupid chase sequence that lasted through all of them, and a few other specific incidents (Don't get me started on the whole Radagast and his rabbit-drawn sled). In this case, a lot of the "added material" aside from the chase and to a lesser extent the whole Tauriel/Kili storyline is stuff that I have no problems with - most of it is from Tolkien, specifically, if my memory isn't playing tricks, the chapter The Quest of Erebor in The Unfinished Tales.
What is it that makes a movie or tv adaptation of a book work for you?