"They didn't want to see me; they wouldn't even [let me] audition," the 26-year-old starlet said of the show's producers. "They thought my image too glamorous, that I couldn't not be pretty."
After reading the script and falling in love with free-spirited Fiona, the eldest of six kids in the unconventional Shameless Chicago family, Rossum took matters into her own hands.
She sent a tape of herself in character to the producers, and they were so impressed that she "flew out the next day" to meet them.
Three auditions later, Rossum had the part, opposite William H. Macy -- and let loose a string of expletives when she found out, which "only seemed to confirm [the producers'] decision."
"I don't let anything send me into a tailspin," she told the magazine of her similarities to the her character. "I never have; it's not in my nature."
But though Rossum explained that she and Fiona share a fierce sense of loyalty, even she admits that she just isn't as blase about some things as Fiona is.
"Fiona is very sexually liberated," she said. "She's also a loudmouth, and there's safety in that. But I'm not the sex-on-the-first-date kind of girl."