Dakota Johnson burst into tears when she was told she had the role in Fifty Shades of Grey.

The actress, who has been cast as Anastasia Steele in the adaptation of E.L. James' erotic novel, said she was thinking about sex the moment she was offered the life-changing part.

"The phone rang and I remember looking out of this apartment to an apartment across the street. And I was looking at all of these people in their windows. And I thought, 'How funny would it be if I were watching people having sex right now?' They [management] said 'You got the offer' and I couldn't speak. I was just so relieved. I was crying and crying. Water was literally squirting out of my eyes because I had been so stressed out. And there's this giant Doberman Pincher laying on the bed next to me. And he just slowly fell asleep. It was so special and [this dog] didn't give a f*** about me and what I was going through."

The 24-year-old daughter of actress Melanie Griffith and Don Johnson will star in the movie alongside hunk Jamie Dornan as her lover Christian Grey.

Dakota said Fifty Shades filmmakers weren't "#####-footing" around the subject of sex during her audition for the role but she finds the story romantic: "I just really understand it. I think it's an incredible love story and that's why it's affected so many people."