Except for Jeremy Allen White, who plays the genius Lip, "all of the kids are only children, which I think is interesting", Rossum, an only child herself, said in a phone interview.
"I feel like you grow up with a little bit of like a loneliness factor, which makes you crave being around big families. Being around other kids makes you basically crave siblings."
On Shameless, Rossum (24) is Fiona Gallagher, the closest thing there is to an adult in a household all too loosely headed by a drunk named Frank (William H. Macy) in the series adapted from a British hit.
And though Macy's Frank is so, well, shameless that you can almost smell him coming through the screen, it's the interaction among Fiona and her Gallagher siblings that's rendered Shameless irresistible.
Working together "was basically satisfying some deeper personal desire that we all had to have siblings and have that sibling craziness", Rossum said.
"So I feel like that side of it came easily to us: the wanting to spend time together, the wanting to bond that would lead to this kind of on-screen believability as a family that have functioned, or dysfunctioned, together for quite a long time.
"I don't consider them child actors or kids," said Rossum, a former child performer who nevertheless acknowledges feeling "a little protective" of her young co-stars.
"They really do work as conscientiously and committedly as an adult. I've been really impressed by them. I mean, little Emma Kenney [who plays Debbie] comes in every week, and she has notes all in the margins" of the script "about the feelings that she wants to play, and she has stress marks" on particular words.
And she's clearly all written it herself. I mean, it's not her parents that are coaching her or doing it, because it's so intuitive to her.
"And little Ethan [Cutkosky, who plays Carl] comes in and just nails every joke," Rossum said.
One question Rossum heard plenty, even before the series launched, involves her character's occasional nudity.
"If the story really warrants [nudity] and you get to be a real character and show them in every facet of what their life is, then I don't think there's anything wrong with that.
"I get asked so many questions, like, `Do you diet during the season?' Who cares?" she said.
- Shameless premieres on Monday at 11pm on TV2.
