Emily Blunt couldn't hide her pregnancy from her 'Girl on the Train' co-star Justin Theroux.

The 33-year-old actress had kept quiet about expecting her second child during the early days of shooting the thriller, but her co-star was instantly suspicious when the British beauty - who give birth to daughter Violet in July, a younger sister for two-year-old Hazel - refused to perform a simple stunt that saw her jump up off the floor and run a short distance across the room.

Justin recalled: ''She was like, 'Yeah, I don't think I'd be sitting on the floor. I think I'd be leaning against the counter.'

''We were like, 'No, it makes total sense for you to be sitting on the floor. And then you just get up and run.' She was like, 'Nah, I'm not gonna do that. I'm gonna twist my hip or something.' ''

The 45-year-old star told Emily he didn't understand why she would refuse the basic stunt when he had previously seen her ''throwing Tom Cruise across the room [in 'Edge of Tomorrow'] and going toe-to-toe with Benicio Del Toro [in 'Sicario'].''

And the 'Leftovers' star - who is married to Jennifer Aniston - had his suspicions confirmed later in the day when he witnessed Emily turning down alcohol.

He told 'Entertainment Tonight': ''Later that day, we went back to my place and she was not having a cocktail and I asked, 'Are you pregnant?' And she was like, 'Shhh!!' And we had a big hug over that.

''It was a little secret we had to keep for a while until it became obvious.''

Luke Evans, who also stars in the movie, previously revealed Emily - who is married to John Krasinski - had warned him about her pregnancy just minutes before they were due to shoot a fight scene.

He recalled: ''In this scene there was a kerfuffle and just before we were about to do it, she leaned in and whispers, 'I'm four months pregnant.' I was like, 'What?' She said, 'I'm four months pregnant.'

''She decided to tell me at that moment so I think I was one of the first ones to find out. But she had to tell me, I was about to throw her around the room, I was like, 'Cheers Em!' But she was a pro and didn't let it affect her.''