It's one of those things where the book has all these stars that burn really bright that you hang onto and they're all saying, 'This is The Girl on the Train experience.' All those stars or hooks needed to be in the film, but sometimes they needed to be a bit different. It's important when adapting such a popular book to hit all those points but also break out expectations without slaughtering the book. And that was, for me, the joy of adapting the book.

I really wanted Rachel [from the Girl on The Train] to be purely fixated on fantasy and on her ex-husband.I didn't want her to be embarking on romance, touching people; I wanted her purely in the realm of fantasy and frustration and dreaming and sadness.

I really needed to dramatize and clarify that Rachel was taking strides towards her own healing and her own sobriety - and that she was actually thoroughly frightened about what she may have done.This was something that was so beautifully done in the book [The Girl on the Train] through inner monologue, but I couldn't just have a whole film filled with inner monologues. So going to Alcoholics Anonymous was a very simple solution to that problem.

It's what you do when you have Allison Janney.My jaw-dropped at her performance. You think that part is sort of a regular part until she steps into the shoes of the officer. She just filled it with such dimension and by giving her the whole shebang, the whole police thread, I think it was the right choice.

It's one of those things where the book has all these stars that burn really bright that you hang onto and they're all saying, 'This is The Girl on the Train experience.' All those stars or hooks needed to be in the film, but sometimes they needed to be a bit different. It's important when adapting such a popular book to hit all those points but also break out expectations without slaughtering the book. And that was, for me, the joy of adapting the book.

I wanted to keep the complexity of the female experience in the film as much as it is in the book, and the subject of not wanting a child is a very interesting subject, one that's not dealt with very much actually.However that complexity was not serving the story of what became the film [The Girl on the Train].

The gaslight of the film [The Girl On The Train] became something that really needed to be dramatized more than the book did, because it wasn't going to read as strongly on screen.