The star, who has played Mia Grey throughout the trilogy, will have significantly more to do in the third film, after only a fleeting cameo in Fifty Shades of Grey and Fifty Shades Darker.
“I was really excited because I definitely had a bigger part in the movie,” she told Elle.
“And I ended up loving each and every one of the cast and crew, including Dakota [Johnson, Anastasia Steele] and Jamie [Dornan, Christian Grey], and everyone was super supportive of one another.
“It was actually kind of sad wrapping up, because I guess that was the last movie, but it’s so exciting.”
Alluding to a darker mood, she added: “It’s definitely way more thriller-y than anything.
“It’s got moments of me getting kidnapped. There’s fighting scenes. It’s really great.”
Rita not only stars in the film, but has also contributed to the soundtrack: For You, which she performs alongside Liam Payne, was released earlier this month.
The music video was released yesterday and teases blink-and-you-miss-it glimpses of Johnson and Dornan in their much-awaited wedding scene.
Fifty Shades Freed is out next month and is expected to dominate the box office upon its debut.
Although likely to be the lowest-grossing entry of the series, it should still do big business: estimates say it could clock in at just under $40 million in its opening weekend in the US alone.
Fifty Shades Freed is out on February 9.