Don't know if when you did that you get a faithful adaptation. At best a film can come close to a book, it can never be the exact book translated to film.
For one, you can not translate an author's voice, the way he tells things, the words he uses, the style he writes in etc.
Secondly, it's been often said, but books and films are two very different media really. For example, in a book you can follow a person's thoughts and thoughtprocesses for pages on end (a whole book really), in a film you can't do it that extensive.
Thirdly the images a book invokes in the reader will be completely personal to each reader. Everyone will have read the same story, but imagined it differently.
I recently mentioned this, but I will repeat it. In King's case, because his books are so much about character, the best you can do is find actors who nail a character (like we've had Sissy Spacek or Kathy Bates) and build the films around them. Those have been the best adaptations.
I wonder if there is a place where Stephen goes extensively into the differences between writing books or screenplays? I haven't come across something like that, but that'd be interesting to read.
He's often (I feel a bit unfairly) criticised for his screenplays, saying he doesn't know what works on screen, but I wonder about that. 'Pet Sematary' I felt was a really good adaptation, which added things not in the book which worked very well on screen.
In general with his screenplays, I think the problem is more that he has worked a lot in the medium of television, which is not ideal for horror, because of many restrictions (budgetary for one, or not being able to put on too much violence, writing around commercial breaks etc.), but he still managed to work around those and produced good work for tv too, like 'Storm of the Century', which is generally well regarded.