The British singer has been cast in the highly anticipated movie, based on the erotic novel of the same name, as lead character Christian Grey's sister Mia.
Rita filmed her scenes for the racy film earlier this year.
Chatting with The Vodafone Big Top 40 this weekend, she said: "[It] took up a lot of my time as I didn't wanna embarrass anybody, so I made sure I knew exactly what I was doing."
Fifty Shades - which opens in cinemas next year - will mark Rita's major movie debut. She previously had a bit part in Fast & Furious 6.
"It was an incredible experience," she said of filming in Vancouver. "I went from not knowing what it's like being on a movie set, to feeling like I knew them all for years, and I wasn't even there for that long.
"They'd already been filming for months and months, so I came in mid-way through the film. I can honestly say it's so cool to be in it. I'm so excited, I was really nervous. I kind of came in last minute, I auditioned last minute."
Rita, 23, announced her casting via Twitter in December.
Fifty Shades of Grey is the first of three erotic novels written by EL James. They focus on the relationship between businessman Christian Grey and publishing graduate Ana Steele.
Northern Irish actor Jamie Dornan will play Christian in the movie, while Melanie Griffiths' daughter Dakota Johnson will play Ana.
Catch The Vodafone Big Top 40 every Sunday at 6pm with Capital presenter Marvin Humes
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