As a lover of both literature and film, I have to say that many adaptations tend to omit parts of the original text based upon time constraints. Also, an adaptation is the director's interpretation of the original. That being said, one would hope that the filmatic adaptation by Polley will be as true to the text as it possibly can given her intense interest in the text.
I agree with you that this film project sounds ambitious. Margaret Atwood’s acclaimed 1996 novel Alias Grace is based on the true crime story of Grace Marks (then sixteen years old) and James McDermott. Marks and McDermott, servants in the Thomas Kinnear household, were convicted of the murder of Kinnear and his housekeeper Nancy Montgomery in 1843. As the title indicates, the novel focuses on Grace; it is told through her voice as well as through letters between Grace and a fictional psychologist who investigates Grace’s claims of amnesia. Atwood focuses on nineteenth-century views of gender and class in developing her story. The letters would not be easy to transfer to film, and as your question implies, many elements of a fairly long novel might be lost.
However, Sarah Polley has been interested in this project for a long time. Astral Media program manager Alan Bacchus claims that it was this project that actually led Polley to want to work behind the camera and not just as an actor in front of it. She received an Oscar nomination for her first film, Away from Her, based on an Alice Munro short story and starring Julie Christie. Polley also directed Take This Waltz, which premiered at the Toronto Film Festival. We can certainly hope that Polley’s passion for this project will result in a good film. Of course, a film is never the same as the book that inspired it, and some people will always prefer one to the other. You will always hear people say, “The book is always better,” but I also think it’s important to recognize that film and literature are two distinct media and serve different purposes. The movie may not capture everything from the book, but it can still become a fine work of art. I am personally looking forward to this film because I enjoyed the book a great deal. I hope Polley does a good job; her track record indicates that she probably will.