Fergie purposefully slumps deep down into couch cushions, leaning forward with one elbow perched on her thigh, a hand dangling between her wide-open knees. “She sits like this,” she says. Then she gets up and makes a heavy, slow lap around a coffee table, letting her hips sway like untied shoelaces in slow motion, her chest thrust, her body letting out an almost audible bah-dum-bah-boom. “She walks like this.”

The Black Eyed Peas singer and solo artist is explaining the unhinged physicality and trashy sensuality of her role as Saraghina in upcoming Rob Marshall film remake of musical “Nine.” Playing a Roman prostitute and outcast, she dances and sings her way into a young Guido Contini’s personal mythology, his first brush with sexuality, at nine years old.

Fergie, who is listed by her real name Stacy Ferguson in the credits to “Nine,” stars opposite of Daniel Day-Lewis (who plays the grown-up director Guido), Penelope Cruz, Judi Dench, Kate Hudson, Nicole Kidman, Sophia Loren and Marion Cotillard. She dances in the sand, dances with sand and sings “Be Italian,” an instructional guide to Guido and the theme that dominates the trailer to the film.

Fergie gained more than 20 lbs. for the hard-fought role, but, she tells HitFix.com, it was good for her. “Oil, butter, fat, salt, sugar, everything. I just stopped working out. So it just started coming on, and I just started getting softer,” she smiles. “It was great. Saraghina actually helped me with body image a little bit. She loves her curves. She’s all about living life to the fullest… [Weight] became a notch down on my priority list, it’s interesting. I dove into playing her so much, that I felt that love.”

Only a few acting stints dot Fergie’s résumé, including a long run of “bad acting” on children’s show “Kids Incorporated” when she was young. But the 34-year-old performer had taken acting classes when she was 24 and finally got around to using the skill set for the screen, and not just the concert stage with the Black Eyed Peas.

“I don’t hold myself the same way in person as I do on stage. It’s larger than life. I don’t have a name for it, like a Sasha Fierce or anything. But when I do a song like ‘My Humps,’ that’s certainly a character. Our shows get a little theatrical,” she says. “Every moment in ‘Nine,’ I was thinking, how would [Saraghina] do this. I don’t think Rob was looking for a person to just sing the song.”

She mentions that Marshall was patient with her as she learned the choreography, a virtue that is rarely afforded in the music world. “That was heaven for me. I don’t get that as a musician. Not to make a cliché but it’s been, well, boom boom…” she stops herself in time and rolls her eyes.

Fergie says she’s caught the acting bug, “the craft,” which may factor in to what she decides to do after the Black Eye Peas finish touring off of “The E.N.D.”

“I don’t know my path at this point. We’re not going to be touring this record forever, we’ll see what happens. I might get inspired and write a rock record, I might get a script… But I do know I’d like to work with Tim Burton,” she says. “I’ve had to really pay my dues up to this point and do everything at once. I don’t want to do that anymore. I’m not gonna take a role that I don’t think I can handle, I’m not gonna take a role if we’re planning a tour.”

"Nine" will be released on Dec. 18 in Los Angeles and New York, and on Dec. 25 everywhere else.

You can read other updates from HitFix's Katie Hasty by following her Twitter, /katieaprincess.