Saoirse Ronan says 'The Host' was a challenge.

The 18-year-old actress plays a girl whose body is taken over by an alien soul Andrew Niccol's new science fiction film - adapted from 'Twilight' author Stephanie Meyer's bestseller of the same name - and she admits juggling both characters was her trickiest acting job to date.

She told MTV: ''It was challenging but it was great at the same time because I'm either playing one or I'm playing the other, or I'll switch between the two in one scene.

''So you'll map that out a bit whenever that happens.''

The movie takes place in an alternative reality where alien beings known as Souls have been implanted into humans' bodies. Rebellious Melanie (Ronan) is one of the last remaining humans, and refuses to give up the fight for her body when alien Soul Wanda takes control.

The Irish starlet relished the challenge of playing two people living within one body, who eventually partner up in spite of their differences to save the human race.

Ronan said: ''I really liked it because for the majority of the shoot, I was playing Wanda, who's - she's a great character to play, but she's a perfect Soul and doesn't really feel any negative emotions.

''So I could go from playing the perfect being to someone who's a real fighter, who's really kinda gutsy, who has a mouth on her, and it was really nice to be able to do that.''