'Look, she's completely comfort-able with her body," says Ed Zwick. "She has a healthy attitude towards sex and sexuality. She's natural about it. It's casual in the best way."

He's talking about Anne Hathaway's uninhibited love scenes with Jake Gyllenhaal in his raunchy comedy romance Love And Other Drugs. She plays Maggie, a sassy artist in the early stages of Parkinson's disease, who embarks on a steamy no-strings-attached relationship with Gyllenhaal's smooth-talking pharmaceutical salesman, Jamie. Not since Kathleen Turner and William Hurt in Body Heat has sex been depicted with such openness in a Hollywood film.

"When people are first in love and are into each other as much as these people are, their bed becomes their world," says Zwick, creator of the ground-breaking 1980s' thirty-something television series that gave a new word to the English dictionary.

"They eat there, they talk there, they sleep there. Everything is there. Had we done the movie with the sheets pulled up to their necks like Rock Hudson and Doris Day in Pillow Talk it would have been very naff. You can't make such a relationship believable to an audience without nudity. It's odd that while people go on about nudity they've no inhibitions about the display of violence, which seems to be more and more explicit and available. There's something wrong about that."

Hathaway has gradually matured from the gawky teenager back when we first met her playing the high-school heir to an obscure throne in Disney's girlie The Princess Diaries. She's always had a lot of spunk, of course. When George Bush was inaugurated as president, she drove, with school friends , eight hours through a blizzard from New Jersey to Washington to join the protest. "I was walking around with braids screaming 'Down With Bush' and stuff like that," she told me.

She's daughter of an Irish American musical actress, Kate McCauley, and was brought up a Catholic. "By the time I was nine I'd seen my mom die twice, once as Fontaine and once as Eva Peron." She campaigned against Bush's re-election in 2006. "My mother threatened to apply for an Irish passport if he won."

But neither her political activism, nor playing a Texan rodeo queen who marries Gyllenhaal in Ang Lee's gay western Brokeback Mountain, which led to an Oscar-nominated role in Rachel Getting Married, not to mention outwitting fashion tyrant Meryl Streep in the sleeper hit The Devil Wears Pravda, suggested she might be capable of the daringly sensual abandon displayed in Love and Other Drugs.

The frank depiction of her bed scenes with Gyllenhaal might be commonplace in European cinema but it's a place Hollywood stars rarely, if ever, go nowadays apart, perhaps, from Natalie Portman, who shows a similar willingness not to hold back emotionally and sexually in Black Swan, a virtuoso performance likely to figure along with Love and Other Drugs in the best actress Oscar nominations.

"Hollywood didn't used to be so cautious, certainly not in the late '60s and early '70s," says Zwick, who has made a speciality of filming relationships, whether in epics like The Last Samurai, Legends of the Fall and Blood Diamond or in smaller films such as his 1986 feature debut About Last Night, an adaptation of David Mamet's Sexual Perversity in Chicago, in some ways a companion piece to Love and Other Drugs.

"The similarities of the two movies are in the close examining of the ups and downs of a relationship. The stakes in About Last Night are more innocent. Rob Lowe's work context is mundane, less complex. The notion of illness Love and Other Drugs describes is a little bit weightier and reflects probably my own understanding of what happens in life, and about health and mortality. But I like to think it is similarly unrestrained in its sexuality and its humour."

"I wonder if some of the caution about nudity today is to do with the proliferation of media outlets, this obsession with screen savers and the kind of promulgation of images that would have been ephemeral in the audience's mind when seen in the movie theatre once but now are forever indelibly archived and available, and subject to manipulation."

So why is Hathaway risking such cyber exposure? "Because I really believed Maggie and Jamie's love on the page and I'd had a wonderful experience with Jake on Brokeback," she tells me.

"I hoped we could get there again, actually with better results, because we didn't really love each other the last time," she says.

"The more talks I had with Ed the more apparent it became to me that it was another adventure worth taking. It's an exploration of love in a relationship. The nudity is really an essential part of the story and shows the intimacy that Jamie and Maggie feel together and how their relationship shifts from sex into love.

For Gyllenhaal, the appeal of his role was the theme of a man coming to a time in his life where he wondered what real love was and if he wanted it. "When I read the script, I happened to be going through a period when that seemed a pressing question. Somehow it moved me to the core, and I couldn't not do it."

He turns teasingly to Hathaway. "Besides, I've always wanted to get naked with Annie again in a movie. I felt like this was an opportunity, so I dived right in. I think I was the 45th actor offered the role after everyone else turned it down because they didn't want to be naked with her."

"So you're saying it was out of pity?" asks Hathaway, with a mock pout. He gets serious. "If you see two people in a movie portraying two people in love, how can you believe these two people are in love if they don't want to be naked around each other. I don't know about you, but I've never had sex with my boxers on, and it's an odd thing to watch an actor doing that. If we were going to be open and intimate about the love story in Love and Other Drugs we'd have to be the same with the sex. So we didn't worry about it. When you're working with someone like Ed you know it's going to be done well."

Hathaway did a lot of research about how her body should look and the effects of medication in the first stage of Parkinson's disease.

"In the majority of cases it causes people to lose weight, so that was the jumping-off point for how I was going to look. But if the medication had the reverse effect I'd have put on weight and still done the nudity."

Zwick took advice from Michael J Fox – a sufferer himself – about the depiction of Parkinson's disease.

"He said you just can't be funny enough about it, and I think that's the way to go. The fact is we're all going to have to deal with the imminence of death, we're just dealing with it at different rates. Every relationship deals with it eventually."

By combining humour with emotion, Love and Other Drugs avoids the pitfall of becoming another Love Story.

"I'm embarrassed to say I've never seen Love Story but from what I understand Ali McGraw has a terminal case of cancer whereas Parkinson's is a degenerative disease and this changes the dimensions of the story," says Hathaway.

Love and Other Drugs is based on true events and set in 1996 when American pharmaceutical companies were first allowed to advertise drugs on television and so directly to the consumer.

It's a satire of the Viagra boom and marketing activities that landed the Pfizer company with an unprecedented $2.3bn fine. But while this provides an ambience, the focus throughout is on the relationship between Hathaway and Gyllenhaal, both of whom give career-changing performances.

"Making the transition from ingénue to leading actress or leading actor can be difficult," says Zwick, who provided Denzel Washington with such a platform in Glory, and also caught Clare Danes, Evan Rachel Wood and Matt Damon at moments when they were beginning to reveal the depth of their abilities.

"Anne is very smart and knows that this would be the time to have people's understanding of her be utterly changed. It's a privilege for a director to be present when this happens."

Hathaway builds on the rapport she had with Gyllenhaal on Brokeback Mountain. "It was great to have someone like Jake as a partner who was so sensitive and respectful during the love scenes and even more so during the emotional scenes," she says.

'I suppose the nude scenes are somewhat out of the norm, but doing nudity is part of being an actor. You're aware that at some point it might happen. We approached it from a very prepared place. We discussed what we were comfortable with. We traded references from other films in order to establish a sort of index of communications we could all share. Some of the intimate scenes required even more trust than that and were even more difficult to sustain for a long period of time and to be able to return to those emotions again and again.

"I'm still learning a lot about how to do my job on camera and off. This job really confused me in a lot of ways. I didn't know how not to take my character home with me. Ed really had to sit there with me and hold my hand and be very patient and talk me through it. And I hate being that kind of actor. I love showing up and just doing my job."

Hathaway's 28th birthday is the next day. "I wish I had more of Maggie's toughness and temper," she says. "I wish I was a more confrontational person. I'm very different to her." She smiles.

"Although," she adds, "I am a Scorpio, so don't cross me."

'Love and Other Drugs' opens on 29 December