Should novelists adapt their books to the screen?
I've performed numerous adaptations. In my experience the person who wrote the book will have too subjective an approach to the source material (and will find it hard to be as ruthless as is required by the adaptation process) to adapt their book into a screenplay well.
Like creative ego (and we're all guilty), this subjectivity is a problem. When adapting a book to the screen, the screenwriter's objectivity is gold dust. The screenwriter's dramatic instincts should also be such that he or she will perform the adaptation with zero attachment to the source material. This will free the screenwriter from shackles that will likely bind the novelist who tries to adapt his or her book to the screen.
It's also likely that the story of the novel can be improved. I have come across this several times when performing a book to screen adaptation. It's the screenwriter's duty to improve the story if possible, while being as kind to the novelist as possible! Politics will also come into play if the novelist is well known.
When adapting a novel to the screen then, objectivity is as essential as the absence of egotistical attachment to the novel adapted. This freedom should enable the screenwriter to improve the story as much as possible, without losing the qualities that made the novel a success. Indeed, a good screenwriter will amplify these qualities during the process of adapting the book to the screen.