What is important when you adapt a book?

It is very important to make as little changes as possible when filming a book. This is very hard, as the time of a movie is limited, every second costs money, and scenes might not be as interesting when absolutely everything is kept original. But no matter what, it is important to keep the basic ideas and essential parts of the book as a identical as possible in the created movie. In addition, the movie should be interesting and entertain.