Firstly, I think the synopsis of Incendiary (before the reviews), where it describes Michelle Williams' character as a 'promiscuous adulteress' is way off the mark. Although she does have an affair, this seems to be something she enters into after much hesitation and is in no way promiscuous. I found those comments in the synopsis a bit harsh and mildly misogynous.
This movie takes a route that's quite different from the book, which (I thought) concentrated more on the erosion of civil liberties by the government and how it manipulates and destroys people's lives, while the film is more about the personal grief of a single person. I found these differences made the original story almost irrelevant to the tale being told in the movie. The story of personal grief told in the film could easily have been accomplished by way of an original screenplay, without adapting the book at all.
Michelle Williams is outstanding in this film, easily the best thing about it. Her accent is very convincing and more that once I got teary watching her performance. The scene where Ewan McGregor's character Jasper visits her in the hospital was particularly moving. McGregor is very good too but I felt his character was sidelined about halfway through and by the end was largely irrelevant.
As for the direction, I thought it at times was too sentimental. For example, the flashbacks of her son on the beach were used far too much. I felt as though I was being clobbered and at one point I said out loud, 'Yes, I get it! Give it a rest!'. I also found the overuse of sentimental music during these scenes irritating. I think this is because I was expecting a different story - ie the one in the book. In the end, the finished film was worthy but not 'incendiary' in the way the book was.