Transition

I believe, that for a page to screen transition to be successful, the screenplay must follow the book very closely. Whats the point of making a movie about a book if the movie is totally different? Three evaluative features of a page to screen transition are keeping the dialogue, keeping events in order and not adding any scenes unless completely necessary.
Dialogue must be kept true to the book because that is the way the author wrote the book and that is the way it should be presented on the big screen. Keeping the events in order is important because the order of scenes greatly impacts the feeling that the movie induces. Last but not least, adding and subtracting scenes would only change the movie and therefore would just be "based on" the book.