I've finished the book now and can report that the movie quite dramatically changed the relationship between Walter and Kitty from the original. They both chart Kitty's growth both in maturity and emotionally but the movie turns the marriage into a late blooming love affair when it never is in the book. Interesting. Basic events remain the same but the movie fills in a lot of detail, particularly to do with the cholera epidemic-- treatment, causes, cultural impediments etc--which is very interesting and adds to the story, really. We learn much more about the character of Walter through his work. In the book Kitty comes to admire him greatly for his skill and compassion but says several times she could never love him.
I think this is a case of a movie enhancing a book but adding the love affair part was probably a marketing device. Kitty's spiritual awakening isn't such a drawcard perhaps!