Though Emily Blunt has played a sword-wielding soldier (Edge of Tomorrow) and a gun-toting FBI agent (Sicario), she says her unhinged alcoholic Rachel Watson, star of the forthcoming The Girl on the Train (out Oct. 7), was “the most exhausting” role she’s had.

In large part, she attributes this to being pregnant with second daughter Violet, who was born in June, for much of the shoot. “It’s actually physically quite tiring to play someone who’s drunk and frightened and unhinged the whole time,” Blunt says. “I think what was the most challenging about it is to play someone like this in the first trimester.”

While she and husband John Krasinski (The Office) didn’t make the news public until late in the film’s shoot, her costar and longtime friend Justin Theroux (The Leftovers), who plays Rachel’s ex-husband, Tom, noticed something was off early on. “We were rehearsing a scene… and she had a stunt — I wouldn’t even call it a stunt,” Theroux remembers. “She was on the floor, and she makes a move to run out of the kitchen, and she was like [to director Tate Taylor], ‘Yeah, I don’t think I want to… I think I’d be standing, and I’d just bolt for the door.'” Blunt looked to Theroux for support, but he was puzzled by her choice. “I was like, ‘No, I think you’d be on the ground, and you’d maybe sit up, and then from a sitting position you’d just bolt,'” he says. “She was like, ‘I just don’t want to twist my hip, and then we’re going to have to use a stunt person…'”

This confused him even further. “I was like, ‘Twist your hip?’ This is the person who went toe-to-toe with Tom Cruise in Edge of Tomorrow!” he says. Eventually, when Taylor left the room, Theroux asked her point-blank, “Are you pregnant?” She said yes, and wondered how he knew.

“I was like, ‘Oh my god, you said you couldn’t be in a sitting position and then start to run!'” Theroux laughs. “‘You’re not 80 years old and you don’t have arthritis. So you’re either pregnant, or you’ve gotten really lazy.'”

But knowing his friend’s happy news early did have its downsides. In the film, Rachel can’t seem to stop showing up at Tom and new wife Anna’s (Rebecca Ferguson) home unannounced, or calling them in the middle of the night, so their scenes together weren’t exactly delicate. “Working with a pregnant person, that’s a challenge,” Theroux says. “All you’re wanting to do is nurture them, and all your scenes with them are yelling at them!”

“You can’t help but think at some point, maybe the baby’s hearing this?” he jokes. “When I finally meet her new child, we’ll see how [she] reacts. When she hears my voice and bursts into tears, we’ll know I scarred the child prenatally.”

For more on The Girl on the Train, pick up the new issue of Entertainment Weekly, on stands Friday, or buy it now. Check back to for even more Train – and don’t forget to subscribe for more exclusive interviews and photos, only in EW.

The Girl on the Train
  • Movie
  • 110 minutes