Before I Go to Sleep is the story of a woman who is suffering from amnesia following a traumatic attack, who wakes each day thinking she is decades younger than she really is. Every morning, she must piece together who she is, what has happened to her, and figure out who she can trust.
Thrillers, more than any other genre, live or die by the effectiveness of their endings, and in particular the preservation of the central ‘twists’. The best thing I can say about the screen adaptation of Before I Go to Sleep is that, despite having already read the book (though admittedly it was a while ago) and therefore knowing how the story would conclude, it held my interest and gave me the feelings of tension you would want and expect from such a film.
It was, plot-wise, a very faithful adaptation, in fact. The only real change was the switch from the heroine keeping a written diary to track her memories in the book to her keeping a video diary in the film, and visually this made a lot of sense, as since this forms such a central part of the story, we would have spent a lot of time watching her silently scribbling away.
The casting choices were good, I felt, with Kidman in particular putting in a solid performance as the confused and mentally frail Christine.
I also felt the choice to have Colin Firth portray the villain worked well to preserve the twist, since we all tend to associate him with playing the dashing hero, and that does, at first, appear to be the role he plays in this.
***end of spoiler***
Granted, neither the book nor the film are necessarily anything especially new or ground-breaking within the genre, but as far as thriller conversions from page to screen go, I think this one fared pretty well.