When she was first cast as Fiona, "I think I really tried to stay in character all the time, almost thinking, 'Oh, they'll see I'm not necessarily like this and then I'll get fired,' " said Rossum, laughing, in an interview during the Television Critics Association's winter meetings.
"It was a struggle just to get the job, initially. I really had to fight against the preconceived notions that the creator and producers and director had of me from ['Phantom of the Opera'], other glamorous things I did."
Now, though, "I think I'm so comfortable with turning [Fiona] on and off . . . and understanding the way she works. I've lived with her as a character longer than I've lived with any character, so for me, she's a big part of me. And I can access that. If they said we were starting the next season tomorrow, I'd be OK.
"I'd just get back in that house and get back in that room . . . so I think for me, being able to put her away, put her in a suitcase until we find the next season, is a healthy thing."
Rossum, 26, who'll next be seen opposite Jeremy Irons and Emma Thompson in "Beautiful Creatures," must need more than one suitcase. She's also a singer and a songwriter, and her second album will be released Tuesday.
"It's called 'Sentimental Journey,' and that's kind of what it is. It's covers of standards from the '20s to the '60s, and I made it with my own money. I kind of intended to release it independently, or online, then I got interest from record labels and ended up partnering with Warner Bros. to release it. And it's all the songs that I loved when I was a kid, that I grew up with," said Rossum, who began her performing life as a member of the Metropolitan Opera Children's Chorus, after singing "Happy Birthday" in 12 keys.
"I had never heard of an audition before like that, but a) it's a song everyone knows . . . and it's easy to judge one person against the next, based on a song like that," she said.
Singing in the chorus, she said, "gave me a respect for the material, and I feel a sense of responsibility to prepare it correctly, whether it's a song or a film or a TV show or a character or whatever. Because somebody worked hard to write that. My teacher used to say, 'Puccini worked really hard on this, so make it good.' "
Plus, "there was a sense of joy and freedom that I always felt when I stepped on to the stage," she said. "I was really into escapism, imaginary friends and I had all that. And I didn't come from some traumatic upbringing that I needed to escape from. I was just very of the escapist mentality."
Even walking a red carpet, which some may regard as a chore, brings out her escapist side.
"Dressing up is really fun, has always been fun for me. I love fashion, I love clothes, I love having that Cinderella moment where you get to borrow something and then it turns into a pumpkin and you give it back. And so I think that may have narrowed me to being the obvious choice for roles like Fiona," she said.
This season of "Shameless," which began Jan. 13, is already shaping up to be a tough one for Fiona, who's made "a couple of bad decisions ... that are more like [her father] Frank's get-rich-quick schemes," Rossum said.
And because "she doesn't really want to push appletinis, we'll see her fight through some pretty disgusting jobs," she said, calling it "a darker season."
What I wouldn't give to have the fake-blood franchise for "Spartacus," which embarks on its final season, "War of the Damned," on Friday (9 p.m. Starz), as if determined to squeeze (and squirt) every last drop from the saga before it ends - badly, history suggests - for the rebel slave (Liam McIntyre) to whom Starz owes so much.
Naked women, too, are not in short supply in Friday's season premiere. Always available to wash off a warrior or pile on, like puppies, for an orgy, they are, along with the buckets and buckets of blood, presumably why Starz thinks viewers pay the big bucks.
And, no, I'm not just sulking because Lucy Lawless' splendid Lucretia is, definitively this time, no more. Call me a party-pooper, but the continued emphasis on random boobs and blood ill-serves creator Steven S. DeKnight's vision of a story that parallels our own economic divide, Romans being the fat cats in this scenario, rebels the Occupy Wall Streeters.
Simon Merrells joins the cast as Marcus Crassus, who's older, meaner and, honestly, more interesting than Spartacus, whose superhero perfection leaves me as unmoved as the relentless, stylized carnage.
If the Bard's not exactly naked in PBS' "Shakespeare Uncovered" (9 and 10 p.m. Friday, WHYY 12), he's at least getting out and about, as actors research his plays in a variety of ways that include visits to places William Shakespeare probably only knew from early guidebooks - and other people's stories.
In Friday's back-to-back season premieres, Ethan Hawke delves into the foreboding "Macbeth," followed by Joely Richardson, who enlists her mother, Vanessa Redgrave, in her exploration of the comedies "As You Like It" and "Twelfth Night."
On Twitter: @elgray