The film is a big-screen adaptation of the first of four books in the Caster Chronicles series of supernatural young-adult novels by Kami Garcia and Margaret Stohl.
"I loved the character. When I read the script, I was just taken by her bawdiness, her kind of fun-loving brand of evil. I like a character that has gone over to the dark side, but has a gray area. It was terrifically fun to embrace the Ridley that I read about in all the books and bring her to life," the 26-year-old New York native told United Press International in a recent phone interview.
Rossum said she drew on numerous sources, aside from the novels and screenplay for the film, in preparing to play Ridley, Lena's cousin and a siren with dark powers.
"And also looking a little bit at Wicca and some spell books and trying to associate with that mentality, but what really worked most for me was looking at goddesses and there's a Norse goddess named Freyja and I kind of based Ridley a little around her energy," Rossum said. "All jobs are [educational.] You learn so much about different walks of life and different things about the world you didn't know."
She said the sexy costumes she wears in the movie also helped her transform into Ridley.
"It helped me get into character immediately because [the clothes] are so full of personality and there was such a kind of 'in-your-face' attitude in the wardrobe, hair and makeup. It was incredibly fun," she said.
Told LaGravenese described her as "fearless" in her portrayal, Rossum graciously replied: "I think I'm just more trusting. I will almost do anything a director tells me to do because I trust in their vision, if I trust them, and I love taking chances and pushing things as far as they can go. So this character was very fun with the camp element to her personality and how over-the-top she is with her self-involvement."
So, did the fact LaGravenese also wrote the film's screenplay make him an invaluable resource on the set when the cast had questions about this new world they were creating?
"I felt like he looked to us to embellish his answers," Rossum recalled. "As actors, it was our job to bring his work to life. It was really fun to collaborate on that. The script went through a lot of different drafts and your lines would get moved around and changed and swapped out. There were times when I said: 'Oh, I miss that line! I loved that.' And he'd put it back again. He was very flexible."
"It was really fun," Rossum said. "Everyone was talented from the people who are younger than me to Emma Thompson. It was really an incredible experience and really fun."
Rossum said she was aware of the "Beautiful Creatures" books because her friends were fans of them, but admitted she hadn't read them herself until she was considering signing on for the film adaptation.
"After I got the script, I read the first book. And then for my audition, when I got the part, I read all the books that were available and I just finished the most recent book, the last book, over this Christmas vacation," she said.
Acknowledging how the character of glamorous Ridley is vastly different from Fiona, the hard-working, tough-talking young family woman she plays on "Shameless," Rossum insisted she wasn't consciously seeking a change of pace by taking the side job of "Beautiful Creatures."
"No, I was just looking for something that inspired me and this character, I thought, would be a tremendous amount of fun to play," she said.
Since the shooting schedule for the movie overlapped a little bit with that of her TV show, the actress found herself juggling two roles at the same time.
"It was a little bit confusing going between both characters, but I just had folders for each character and I would refresh myself as I'd go back and forth on the plane," she noted.
A former member of the Metropolitan Opera Children's Chorus, the busy, beautiful Rossum still makes time for music in her life. She released a collection of standards called "Sentimental Journey" last month.