Marcia Gay Harden says she isn't allowed to be "overtly sexual" whilst promoting 'Fifty Shades Darker'.

Marcia Gay Harden

Marcia Gay Harden

The 57-year-old actress stars as the mother of the S&M loving hunk Christian Grey in the upcoming second instalment of the 'Fifty Shades of Grey' film franchise, and has said despite the movie's often raunchy nature, the cast have been banned from talking about the sex scenes by their bosses at Universal.

She said: "We can't talk too much about nipple clamps.

"It is a love story after all -- I don't think they want us being too overtly sexual in interviews."

And the Oscar-winning actress says she's had a hard time keeping her lips sealed about the steamy scenes, and has found herself in trouble with the movie's bosses for things she's put out on Twitter.

She added to The Sun newspaper: "I used to send out some naughty little tweets, along the lines of this 'sweater clasp' and I was told by Universal that I couldn't do it any more, but I'll tell you one anyway.

"I'm a good girl, I fell in line. Because it's supposed to be more about the 'romance'.

"Maybe I went too far ... I don't know.

"I sent out a picture of a sex toy and I was like, 'Dear Christian, thank you so much for that lovely bracelet under the Christmas tree. Perhaps it wasn't meant for me. Oh well, so pretty.'

"And all the fans would write back, 'No that wasn't for you Mama Grey, oh no.' That was a lot of fun but it was shut down."

Meanwhile, leading stars Jamie Dornan - who plays businessman Christian Grey - and Dakota Johnson who stars as Anastasia Steele, have admitted they became "great friends" on the set of the movie which made filming their scenes together "more comfortable".

Jamie said recently: "I was only cast five weeks before we started filming the first movie, and that's when I met Dakota for the first time. [Now] it's been three years, [and] Dakota and I are very close.

"We've got a great sort of love and respect for each other. We're great friends and I think that makes it easier, you know? So it's definitely more comfortable."