She is not like other girls. “I’m not this super-pretty popular person,” Kat Dennings says, 22-year-old head slung low, silky brown tresses hiding one entire side of her face in a Veronica Lake wave. “Sure, I have my moments,” says the star of the nocturnal comedy Nick & Norah’s Infinite Playlist, gazing curiously down at herself, and at the shimmering black sleeveless mini-dress she’s chosen for today’s movie promotion — though she wears it, slouched forward here in a Toronto hotel room, with the sweetly sullen mien of a teenage tomboy who’s been forced to dress up for daddy. “But if you want to know the truth, I’d rather be at home in my pyjamas.”

Pyjamas, it soon transpires, play a big part in the life of Dennings. They are her outfit of choice while at home in LA, on breaks between filming the forthcoming indie drama Arlen Faber and the new Robert Rodriguez action comedy Shorts. They feature heavily in her regular YouTube video blogs (the true mark of a digital-age star), during which she prances around her bedroom in, alternately, the orange check, grey striped and pink flowery variety. She wore them for almost the entire production of Nick & Norah, changing into her screen denims only when the movie’s gruelling month-long night shoot demanded it.

“We’d arrive on set when most people were going home from work,” she says, describing the late night set-up in the Lower East Side of Manhattan, in November 2007. “And then we’d work all night. Most days I didn’t bother with real clothes. Just my hunting cap, boots and pyjamas.”

The movie, a leisurely hipster rom-com from the school of Juno, follows the eponymous protagonists, played by Dennings and Michael Cera (also from Juno), as they ping about the streets of New York, hoping to find the secret midnight venue for an achingly enigmatic punk band called Fluffy. In reality, of course, these two star-crossed music lovers are trying to find each other.

The narrative may be light, the plot complexities rare, but the casting of Dennings is the movie’s genuine coup. With a sleepy deep voice, half-closed eyes and full-bodied screen presence (Yes, she has real hips! And thighs! And real flesh on her body!) she is a fabulous antidote to the legions of squeaky blonde twiglets that normally constitute the rising-starlet brigade.

“I’m not your typical heroine,” she says, before adding drolly, with a low Lauren Bacall smirk, “But I wouldn’t rule out playing a cheerleader, if the right script came along.”

Typically, Dennings has become the first choice for casting directors in search of bookish, slightly introspective girls with a touch of va-va-voom. She played the wise-beyond-her-years daughter of Catherine Keener in The 40-Year-Old Virgin, and of Robert Downey Jr in last year’s indie comedy Charlie Bartlett (“Robert is just sooo amazing. He’s really smart. We have a lot in common!”). And yet there seems to be direct and conspicuous connections between her on and off-screen personae.

From Philadelphia, and the youngest of five children, she graduated from highschool precociously, at 14, having never once set foot inside the building. Instead, she was home-schooled by her biochemist father and speech therapist mother (“They were unhappy with school system at the time”). Consequently, she remains a heady creation to this day, is close friends with the 28-year-old LA literary tyro Andrea Seigel (To Feel Stuff), and has written her own screenplay — an unnamed and top-secret project currently in pre-production. A voracious reader, she is prone, during the course of our conversation, to extolling the virtues of Japanese writer Haruki Murakami (“He’s like, the Nabokov of Japan!”).

Even at 14 years old, however, and free from the obligations of education to pursue a career in acting, she says that she could see straight through the entertainment industry. Her first gig was a commercial for the now defunct Orleans low fat crisps, or “potato chips”. “They were like poison, and we weren’t even allowed eat them for the commercial,” she says. “They’d open the bag, and then dump them and replace them with other chips. It showed me, right then, how this whole industry is full of crap.”

She nonetheless became a big draw and, playing up or down the soulful, sullen routine (big eyes, lots of mascara, pale skin, and greatest full-lipped pout since Angelina Jolie), graduated from bit-parts in CSI, Sex and the City and ER to supporting roles in Down in the Valley and The 40-Year-Old Virgin.

Her big scene in that last movie, bursting in on mom Keener and partner Steve Carell in bed is already a classic clip (“I can’t believe it!” screams her character Marla, cornering a semi-clad Keener, “You’re allowed to have sex and I’m not! That is soooo unfair!”).

Today, as the star of Nick & Norah, and four forthcoming movies of impeccable pedigree (including the Don DeLillo adaptation End Zone) Dennings is struggling with the inevitable arrival of the high life. Though she was previously romantically linked to fellow Hollywood actors Ira David Wood (Down in the Valley) and Matthew Gray Gubler (The Life Aquatic), she announces firmly: “I don’t have a boyfriend. I have friends, girl friends, great friends. We just hang out in other’s places and have one-on-one bonding time.”

And as for the impending arrival of interstellar status? “If it were to happen, I don’t think anything would really change about me,” she says, before adding, quick as a flash, and with a wry nod to her hermetic pyjama-clad lifestyle, “I’d just, finally, put on some normal clothes and go outside.”

Nick & Norah’s Infinite Playlist is released tomorrow

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