The 'Melancholia' star said that although Mortensen's casting convinced her to accept the part, she was nervous about working with him, reported Harper's Bazaar magazine.
"He's one of those actors. The first time I met him he was very reserved. It was in an elevator. I was like, 'Hi'. He was like, Oh hi.' I learned later he was very shy. So I was nervous, even a little scared, to work with him. I thought, 'This is going to be intense," she said.
"I laughed so much with Viggo... I'm sure he wouldn't be happy with people knowing how fun he is," she added.
The movie will be releasing in the UK on April 16.