Justin Theroux guessed 'lazy' Emily Blunt's pregnancy while filming

Justin Theroux accurately guessed his The Girl on the Train co-star Emily Blunt was pregnant after she uncharacteristically became "lazy" with her on-set stunts.

The Sicario actress stars in the highly-anticipated film adaptation, in which her real-life longtime friend Justin plays her ex-husband.

As they already knew one another very well before shooting, Jennifer Aniston's husband tells Entertainment Weekly he had a hunch something was up with Emily when she was keen to make changes to her action scenes.

"We were rehearsing a scene... and she had a stunt - I wouldn't even call it a stunt," Justin recalls. "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.'

Emily looked to her pal to support her choice, but he was still confused as to why she was refusing such a simple move.

"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.' She was like, 'I just don't want to twist my hip, and then we're going to have to use a stunt person...'"

Justin admits he was even more puzzled by her comments, continuing, "I was like, 'Twist your hip?' This is the person who went toe-to-toe with Tom Cruise in Edge of Tomorrow!"

When the director left the room, Justin wasted no time in finding out if Emily was pregnant, and the mum-to-be wanted to know how he had figured it out.

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

Once her secret was safe with Justin, he became even more conscious of Emily's well-being, something he found difficult to reconcile.

"Working with a pregnant person, that's a challenge," Justin confesses. "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? 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."

Besides filming while pregnant, Emily also struggled with playing drunk.

"It's actually physically quite tiring to play someone who's drunk and frightened and unhinged the whole time," Emily explains. "I think what was the most challenging about it is to play someone like this in the first trimester."

Emily gave birth to Violet, her second child with actor husband John Krasinski, in June (16).