The singer and X Factor judge plays Mia in the erotic film franchise: her character debuted in the first movie, released in February, but has a larger role in its sequel, Fifty Shades Darker, which is scheduled for release in 2017.
Rita told Billboard: "I love all three books and for the fans that have read Fifty Shades of Grey part two, they know it will definitely be a massive growth. I'm so happy to be part of this franchise and be part of this transformation of the imagination of sex."
Her character Mia is the sister of protagonist Christian Grey, played by Jamie Dornan. Mia's role becomes more important in Fifty Shades Darker and final book, Fifty Shades Freed.
She told Glamour earlier this year she'd loved her time on the first movie's set and couldn't wait to get started on the next film.
"It was great because it's a really cool ongoing sequel, so it's only gonna grow, and this was honestly such a quick cameo," she said. "And if you read the novel, my character is really actually in it for that amount of time. Moving forward, it's gonna be very exciting to get back into it.
"I'd love to do more acting, I think it's something I'm definitely passionate about. It has to be the right role, the right story."