The Reckoning: the Good
Changes from the book, which (IMHO) makes the film better than the book.
If the ending of the book works better than the ending of the film, than the beginning of the film works better than the beginning of the book.
The reason Nicholas ran away in the book differs from the reason in the film, but, as most of today's film audience would most likely not understand the reason in the book, a new reason had to be found for the film. Thus, the reason for the adultery in the film.
When Nicholas first meets the actors, all are more active in the film, than they are in the book, where they are more passive.
In the book, the actors are just actors, though Stephen and the woman are in a common-in-law marriage, but in the film, they are not only actors, but also kin. The dead actor is Martin's father, the woman is Martin's sister, and the boy is Martin's son.
In the book, the dead actor died of a stroke, but in the film, he is the victim of a mercy killing, at the hands of his own son.