Natalie Portman with Ashton Kutcher in No Strings Attached

It’s a very good month to be Natalie Portman. With a Golden Globe in hand for her performance in Black Swan, the petite star is about to do something more surprising than scoop up a major prize on the road to the Oscars. She’s going to tackle big-studio, R-rated comedy.

And hearing the raucous laugh that rips from Portman’s mouth as she tries to fit in a photographer’s frame with co-star Ashton Kutcher, it starts to make sense.

“You really chose to wear flats?” Kutcher teases, towering 10 inches over his 5-foot-3, pregnant co-star at the Four Seasons Hotel. “I know!” Portman says, gingerly stepping onto a wooden box and folding her hands over her growing belly.

A few minutes later, the two plop down on a nearby couch, and Portman, 29, shows off her shiny black ballet flats. “Comfort counts, for so much more than anything,” she says.

Together, she and Kutcher, 32, banter easily — the joint interview is a chance to see the two private stars share big laughs while revealing a new side of themselves.

And for Portman, that may be the point.

In No Strings Attached, she plays Emma, a workaholic doctor eschewing relationships and pulling 80-hour workweeks. The kink in her plan: she challenges Kutcher’s character, Adam, to the male dream — a round-the-clock, sex-only, no-strings-attached relationship.

“This is the kind of movie I like to go see more than anything,” says Portman, who calls the romantic comedy a “palate cleanser” after her dark turn as a mentally unstable ballerina in Black Swan. But choosing something different was key for Portman, known for heavier roles in V for Vendetta, Brothers, Closer and The Other Boleyn Girl. In fact, her schedule after Swan is all over the map: she shot a turn in Thor (out in May) and fantastical comedy Your Highness (out in April) back-to-back last year with Strings.

“I’m not interested in repeated experiences,” she says. “This is the first time I read a script where the female character was as funny as the male character and as specific, and as much of a real person.”

And she leaned on Kutcher, a comedy vet who has wooed the likes of Katherine Heigl, Cameron Diaz and Jessica Alba on the big screen in Killers, What Happens in Vegas and Valentine’s Day. For Kutcher, Portman and the brash script sold him immediately.

“I see a lot of the Judd Apatow movies that I really like and they’re like this honest, male perspective on relationships, rated R, funny,” Kutcher says. “And I hadn’t really seen a movie that approached it equally honest from the female side.”

No Strings Attached delights in its R-rating. Dirty jokes fly from Portman’s mouth at pirouette speed. Kutcher, playing the son of an ageing TV star (Kevin Kline) trying to make his mark as a writer on a Glee-like television show, sends texts instead of flowers, booty calls instead of love letters. The two strip off their clothes more than 13 times, with Portman’s Emma calling the shots.

“It’s naughty in a kind of original way,” says director Ivan Reitman, who describes Portman’s guffaw as “the bawdiest laugh I’ve ever heard.”

No Strings Attached, he says, is a fresh look at the new rules of dating, the kind of romantic comedy where a sparkly ring really isn’t the point. And gone is the harpy woman of comedy. Emma’s not bitter, “just busy” says Portman, who signed on three years ago to executive-produce and star in the film.

Bare all

Portman hopped into “funny, awkward” sex scenes with Kutcher dressed in modesty bras, body makeup and strategically placed sheets.

And Reitman says her strict Swan diet melted away. “One of the things I love about her is she likes her food,” says the director, who joined in on cast dinners at Portman’s Los Angeles home. “Her boyfriend — soon-to-be husband — is just a great cook. She was proud of that fact.”

In kind with the role reversal, it’s Kutcher’s bare backside you see more than Portman’s. A body double, surely?

“No, that’s his fine butt,” Portman says with a giggle. “I probably should have (used one) but I didn’t,” Kutcher says. “You’re wearing a towel (in those scenes). Also, I didn’t get in shape for this movie so I was really paranoid. I didn’t work out at all.”

This, from the guy who is on the February cover of Men’s Fitness? Portman raises an eyebrow. “Like your ‘not in shape’ is every other guy’s dream.”

“Photoshop,” he says bashfully. “So I was just a little, like, not feeling good about myself on those days (on set).” “Aww,” Portman spouts girlishly, mock-concerned about his body image. “Ashton, we should talk about this more.” He grins, covering his face with his hands.

Reitman says their on-and off-screen chemistry was instantaneous. “We would spend half our time just spit-balling ideas for the film,” which once bore an unprintable title, Kutcher says as Portman chortles on the couch next to him. “We were just throwing out stupid titles,” he says. “We just had a blast.”

Baby on board

Bed-hopping and ballet aside, the little belly popping out of Portman’s Jason Wu frock is the scene-stealer today at the Four Seasons. Portman’s pregnancy and impending marriage to Benjamin Millepied, her Black Swan choreographer, who Portman has said “partnered me in the movie and who now partners me in life,” took celeb-watchers by surprise. She’s now out of her first trimester, and the star’s frequent appearances on endless awards season red carpets presents a conundrum.

“It’s the most private of experiences, so I’d definitely rather be at home right now. But also, it’s my job,” says Portman, her hands naturally resting on her belly as she speaks.

But she’s also picking up A-list advice, such as at a recent Hollywood meet-and-greet.

“The thing that was nice was hearing how like Amy Adams and Nicole Kidman were both talking about how they choose roles so differently (now),” Portman says.

Plus, she says, she has friends like Kutcher, calling him “a very good parent.”

The stepdad of Demi Moore’s three girls, Rumer, Scout and Tallulah, doesn’t think he qualifies in the baby department. “Tallulah was like,” he counts on his fingers, “8 when we (Demi and I) first started dating. So I’ve got like the 8-to-20 thing on lock.”

Portman nods. “Especially with girls, that’s super-hard.”

“Your best teachers are your parents anyway,” Kutcher says. “You take the ‘best of’ from your parents and you try to eliminate the ‘worst of’ from your parents. It’s completely a learn-on-the-job thing.”

And don’t go asking if he has baby fever. “I don’t know what that is,” he shrugs.

What he does have, Portman reveals, is a real connection to Judaism. “Ashton has taught me more about Judaism than I think I have ever learned from anyone else,” says Portman, who was born in Jerusalem. “Ashton’s a very serious student of Kabbalah and Judaism. He knows a lot.”

Ashton interjects: “I can make the noises. I don’t know what they mean.”

“No, you like — he reads the Torah every Saturday,” she exclaims, and then realises she may have revealed too much. “I hope this isn’t too private to reveal, but it’s very impressive.”

Kutcher grows quieter. “I’m a spiritualist, so I study a lot of different spirituality and try to understand it, and understand where people are coming from.”

Off-set

Out of the spotlight, both Portman and Kutcher run their own production companies. Her HandsomeCharlie Films is producing the upcoming Pride and Prejudice and Zombies, along with Important Artifacts, her next project, which Portman says will co-star Brad Pitt.

But that’s “a long time off,” says the star, who will take a break after promoting Thor and Your Highness. “I have no idea what this is going to be like,” she says with a nod to her belly, “so I’ll feel it out.”

Kutcher is shooting new romantic comedy New Year’s Eve, and with his Katalyst Films production company might be planning a reboot of his signature celebrity pranking series, Punk’d.

And he’s working on landing Justin Bieber as host.

“Whoa,” interrupts Portman. “That’s cool. The Biebs? You got the Biebs?”

“Maybe,” Kutcher says. “If I can get him off tour.”

The interviewer resumes a line of questioning, but the co-stars are whispering on the couch.

“You’re the first on the list, by the way,” Kutcher whispers.

Portman stares him down.

“You know I’ll never speak to you again if you punk me.”

“I know,” he says. “I’m prepared for that.”