''There was a bit of pressure,'' the garrulous 29-year-old admits while promoting this year's opening film at the Toronto International Film Festival.

''When we met, Rian said, 'I really see this girl and her story with her son as the emotional heartbeat of the film.' Certainly it's where the big moral questions come up, of nature versus nurture and the redeeming qualities of a mother's love.''

The film starts out with Joseph Gordon-Levitt (The Dark Knight Rises) as a so-called ''looper'' who must kill his older self, played by Bruce Willis, after Willis is sent back in time from the year 2074.

Blunt describes the film as turning from a Blade Runner-type scenario into a movie like Witness or The Good, the Bad and the Ugly. With her as the gun-loving Clint Eastwood, of course. ''It was a challenging part, so I really wanted to embrace the physicality of it.

''Vanity was all stripped away, as she is this weather-beaten midwestern girl.''

The pizza-loving British actor has had to forgo her favourite food of late as she is getting in shape for her next action film, co-starring as a highly decorated soldier alongside Tom Cruise in All You Need Is Kill.

''I've been doing martial arts and gymnastics, track, yoga and strength training. It's like a combination of torture,'' she says.

Even looking buff and taut, Blunt admits she has been struggling to keep up in her rehearsals for her fight scenes with co-star Cruise. ''I was like, 'Wow, this guy's a machine!'

''Tom's been doing this for so long and yet he hasn't lost his excitement about it. He's better than most stuntmen, than nearly all of them, actually.''

Looper is out on September 27.