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.

Just because Rachel [from the Girl On The Train] is an unreliable character doesn't mean she has sex with anybody who walks by her. It was important to keep her a little virginal.

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 needed to enhance the outward threat to Rachel.In the book [Girl on the Train], her inner threat is so strong; the fear of herself and her inability to remember and the false memories. In the film [Girl on the Train]I wanted to increase the exterior threat. So that's why Allison's part was bigger and was an important part of the climax of the film.

I always thought that the location of this film [Girl In The Train] was on the train and inside her imagination, and her loneliness and her gaze out the window.Although it was set in England, it didn't feel to me like an overly English book. In terms of the use of cultural references, it was not extreme, so it was very simple to go from England to America in the adaptation.