American actress Dakota Johnson who is set to play Anastasia Steele in the film version of EL James' erotic novel Fifty Shades of Grey has described the book as "an incredible love story."

In the film, the 24-year-old daughter of actors Melanie Griffith and Don Johnson, will be seen opposite Irish actor/model Jamie Dornan, who will play Christian Grey.

"I just really understand it," Johnson told Entertainment Weekly, "I think it's an incredible love story and that's why it's affected so many people. Erica (EL James) did a really good job of explaining how that just can happen sometimes and you have this chemical pull to someone. Adding in the sex makes it perfect. Sometimes you feel a little bit naughty and that's okay."

Johnson also revealed her excitement on landing the role.

"They said 'You got the offer' and I couldn't speak," she recalled, "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."

Meanwhile, the first photo of Dornan and Johnson as Grey and Steele was released on the cover of Entertainment Weekly.

The 31-year-old actor is seen hugging Johnson, with a grey tie entangled on his hand.

"I'm a fairly worldly guy. I grew up in a very liberal place. I'm not saying we had a playroom, but I'm not shocked by [the sex in the book]. It's essential to tell the story. I can't believe films that don't invoke the sexual side of it. So it works for me," he said of his character.