Of all the notable characters in the movie, only one is a total Wilson invention. Martha (Lisa Kudrow), the wife of Tom's former boss, isn't in the film much, but her appearances are crucial: The sight of her in passing causes Rachel to flashback to a party where she'd apparently made a drunken spectacle in front of Martha and Tom's work colleagues. And an actual encounter with Martha allows Rachel to re-examine truths behind her alcoholism, her marriage, and how both are linked to Megan's disappearance.
"The gaslight of the film became something that really needed to be dramatized more than the book did, because it wasn't going to read as strongly onscreen," Wilson said of Rachel's slow realization that Tom was manipulating her. "I think that you needed to be hit harder with that information, and Tate had this idea to just have Lisa Kudrow literally say it. No more beating around the bush. I think there are moments where it's right to do that, and that was the moment. To take an element of the book that was already there and turn it into a living, breathing character in Lisa."
That second scenario also offers an unexpected twist for fans of the book: Rachel's shocking golf club attack from the book is actually committed by Tom in the movie. "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."