The 'R.I.P.' singer has her first big-screen speaking part as lead character Christian Grey's sister, Mia, in the hotly anticipated erotic thriller, and promises audiences they will be surprised by the content of the movie.
She told MTV: "It's gonna be the biggest and most amazing shock ever."
Rita has previously had acting roles in 2004 independent film 'Spivs' and a small part in 'Fast and Furious 6', but this is the first time she is to appear in a major production with a full role, and admits she was nervous.
She said: "I was nervous because it was the first time I ever did a movie with words. But it was so nice. The director, Sam Taylor Johnson, she was incredible, so it was all love."
Rita beat Rihanna to claim the part in 'Fast and Furious 6' which could be one of the reasons the 'Bottom's Up' singer reportedly snubbed her at various Grammy Awards parties over the weekend but spoke to her boyfriend, Calvin Harris.
A source said: "Rihanna deliberately kept her distance. She talked with Calvin at length but didn't acknowledge Rita.
"It was awkward because they have the same management team and move in exactly the same circles.
"Rihanna's never been much of a fan of Rita because her style is so similar and they compete over songs."