It was good on the radio
Stella Gonet's reading of the book in five episodes for the BBC, is excellent. <http://www.bbc.co.uk/programmes/b0125hs8/episodes/guide>
The mark of a good thriller is usually that you don’t guess the who or the why or the what of it. Or, if you do manage to guess the who, why or what, at least you shouldn't be able to guess the how. But sadly, all that can be undone, if, after the big reveal, your first thought is: "But wait a minute, then why didn’t X and Y …
Watch The Guest instead. I've seen both films and The Guest is much better. Dan Stevens is a complete revelation and the daughter gives an excellent performance. Before I Go To Sleep has a small number of possible outcomes, one of which happens. It is very predictable. But you won't see the twist coming in The Guest. Sure, it could be better, but it gets a clear 4 stars as opposed to BIGTS's 3 stars.
"you’ll realise that the whole thing is one of those situations that depends on certain key individuals not saying anything about things you’d kind of expect them to talk about"
Sounds like the "Harry Potter" films then, where most of the messes could have been avoided if Harry, Dumbledore and others hadn't been so fond of keeping secrets.
I read somewhere that Hollywood films are often shown to a trial audience or two before general release. Comments and feedback are taken from the popcorn-munchers and often as a result the film may be re-edited, partially re-shot or even re-written.
You do wonder why we all expect more from this sort of thing, when in reality its creative drive originates in the spreadsheets of a vertically-integrated entertainment business.