Emily Mortimer is laughing. 'Bianca's a very subtle actress and incredibly modest. She sits quietly, she waits. She doesn't pace around, throw hissy fits or demand things. And her performance is incredibly low key.' Bianca is Mortimer's Danish/Brazilian co-star in flaky new comedy Lars and the Real Girl. Bianca also happens to be a life-sized sex doll and the object of all consuming love from introspective oddball Lars Lindstrom, played by gifted character actor Ryan Gosling. British actress Mortimer is the protagonist's bemused but caring pregnant sister-in-law, Karin, whose deepening friendship with Lars' rubber paramour proves to be one of the many charms of this languid psychological comedy drama.

For Mortimer, a nice girl from London, the daughter of renowned barrister-turned-novelist John Mortimer, this was her first experience playing opposite a sex doll. Was she able to keep a straight face? 'That was the weird part. There was a very respectful vibe around her. Ryan and the director [Craig Gillespie] made sure it was a no-go to take the piss out of Bianca. You had to be cool. But your natural instinct wasn't to do that anyway. There's something about this doll that makes you look at yourself.'

Written by Six Feet Under scribe Nancy Oliver and also starring US independent cinema queen Patricia Clarkson, Lars and the Real Girl is unusual both for its setting – a small Midwestern religious community – and the fact that redemption for its quiet, troubled central character comes in the form of a custom-ordered lady doll. 'It's an archetypal story about somebody dealing with feelings of loss and grief and loneliness, an inability to cope with the world, feelings of rejection and neglect, and acting out,' Mortimer says. 'And then how he eventually comes to terms with those feelings. It's a story we're all familiar with in our own lives as well as in fiction. But what makes this fresh and strange and different and breathes new life into that old story is the presence of Bianca. She breathes life into it even though she's an inanimate creature sitting there amongst us.'

Mortimer, whose previous roles include turns in Scottish features Young Adam and Dear Frankie now lives in New York with her actor husband Alessandro Nivola. She says, 'The absurdity of life can be very funny so the tone of this was sometimes hard to gauge. And the lines were so brilliant and often hilariously funny, so you had to resist the temptation to play the joke.'