As the eldest daughter of the recession-depressed Gallagher clan, Rossum's character is charged with raising her nine younger siblings while her father (played William H. Macy) goes on a permanent bender.
But while the concept – based on a British show of the same name – may sound downbeat, "It's really outrageous and funny, the way it's told and depicted," the Phantom of the Opera and Poseidon actress, 23, said at Sunglass Hut in NYC, where Rossum helped launch Vogue Eyewear's CFDA Capsule Collection.
"Nothing's taken too seriously. So even though it's a hard circumstance, the family has an incredible will to survive. And there's a lot of funny naked drunkenness."
As for baring it all onscreen, "It's new for me," says Rossum, who shared the experience with real-life friend and costar Justin Chatwin. "I was really comfortable with Justin. He's kind of like my brother. Well, I shouldn't say he's like my brother. But we're that comfortable with each other, so it didn't feel weird."