When you adapt a novel as detailed as this one, you have to make some choices. For example, the book goes into great detail about the heroine, Louise, and her past. We decided to leave that part out. But we still wanted the audience to understand her place in society. I’ve lived in Paris for 15 years, and I think the daily commute between Paris and the suburbs is a key detail.

And it’s a real marker of social class. In the book, the daily commutes is what tells you that Louise is lower class. I also liked the idea of slowly revealing her living situation as the movie progresses. You start by imagining her making the trek to a housing estate in a far-flung suburb. But then you realize she’s living in near squalor.

I wanted to film a train because they’re so visually appealing—they look great on screen. A train scene adds movement, which is important in a movie about someone’s state of mind, about madness. I’m thinking of one shot in particular: our character is on the platform, lost in thought, and the RER thunders by. The train lends a power to a scene that’s static otherwise. That let me place Louise, played by Karin Viard—a well-known actress—as one character in a crowd of strangers.

To make her character believable. I didn’t want the audience to judge or condemn Louise. Filming her on a train is a way of putting the audience right there in her daily life. They can identify with her, because most of us have to take public transportation to get where we’re going. It was a way to make her character relatable, not just a monster, a child killer.

Professionally, yes. And what I really loved was filming on location. We brought in some extras because I wanted to capture people close up, but it was nice to be able to get the crew aboard one of the cars, to catch things as they happened. The shoot felt a little bit like a documentary.

We had to change trains halfway through because there was a billboard that wasn’t supposed to be in the frame. Then the next train was delayed, so it got a bit crazy, but I never felt put out. When you make a film, you have to accept certain things. Like the weather. And a lot of scenes were filmed on a closed set, so when we got to shoot outside, it was good to go with the flow.

We only had a half-day to film. Ideally, we would waited for sunset to catch that golden hour lighting. But we were ahead of schedule, so we filmed that scene late in the afternoon. And lo and behold, we got a shot where Louise is daydreaming about coming back from vacation and that unexpected ray of light flits across her face. We never would’ve captured that if we’d boarded the train later in the day, as originally planned. So we made the most out a rather tumultuous shoot. It was a joy.

When I moved to Paris, I lived above the Magenta station. The E line had just opened and I thought it was ever so modern. I’d ride it to line 13, which took me to my university in Saint-Denis. So I was very familiar with it, but had never taken it all the way to the end—out to the suburbs where Louise lived. I wanted to be faithful to the book in not letting the audience discover where she goes when she takes the train home. Basically, we just wanted to show that no one with limited means can live in Paris anymore.

There’s an amazing shot in “High Life” by Claire Denis. The film tells the story of a group of outcasts shipped off to a kind of prison in space. They remind me of the hobos you find in American stories, vagrants that crisscross the country, hopping trains. There’s a flashback where you see the faces of the rough young drifters. And then suddenly, there’s a shot of an oncoming train, like it’s going to run you over. It gives you a sense of impending doom. But before that, there was Hitchcock.

In his films, trains create anxiety and suspense because there’s no escape. It makes me think of that amazing Korean film, “Snowpiercer”, where the train is also cut off from the world. It’s no surprise that so many films feature trains. The attraction is magnetic—the lines buzzing overhead, the rails racing by... It’s crazy how much the rails resemble actual film, always rolling. As François Truffaut said in “Day for Night”, “Films move like trains, you know. Like trains in the night.”

I love them! Travelling by train is a special time for me. I can work, or if I’m with the kids, I can give them my full attention. I also enjoy long trips. I took the Trans-Siberian all the way to Vladivostok and never got bored. Time is suspended—those can be precious moments. You can watch a movie, read, write. It makes me sad that smaller lines have closed and people can’t get around because petrol is too expensive. I really hope that the green transition changes that—that it lets us reopen stations and get the lines back up and running.