Speaking to TV Line, Rossum explained that negotiating herself out of the sex scenes and nudity in the show was "not an option".
"I had to fight for this role [of Fiona] because I definitely wasn't an obvious choice," she said. "I was living in the West Village at the time of my audition and it was raining and I remember thinking, 'I should take a cab'.
"Then I thought, 'No, screw it - Fiona wouldn't take a cab. I'm really going to do this thing'. My lack of vanity in the audition hopefully showed that it was very important to me to portray the character in a real way, as this raw person."
Rossum explained that she sees the sex scenes in the show as part of that, saying: "When they told me about the nudity and the sexuality, I've always been hesitant to do it in the past, but this is a classy group of people. Plus I knew that Bill Macy would also be showing his ass, so I wasn't worried!
"On a serious note, if you're going to show real people, you need to show their entire lives. Sex and romance are such a big part of an adult life, so to have it in any way that was more glamourised than the rest of the show would be odd."
Shameless airs on Sundays at 10pm on Showtime.