Come back to her place, have a tumble on the bed, then leave if you like. Just don't call her "girlfriend" or it's over.
It's no surprise that he agrees to having no-strings sex, but it is startling that the movie shows the co-stars shedding their clothes in a steamy succession of brief encounters, regularly concluded by Hathaway insisting: "It's time for you to go."
Nudity is a body issue Hathaway has strong views on. In general, she steers clear of stripping off in films, although she has appeared topless previously with Gyllenhaal in Brokeback Mountain, then did an even more explicit nude scene in 2005's good-girl-gone-bad drama Havoc, which ended up going straight to video.
"I don't actively search for movies that I can get naked in. It's not the most fun requirement of the job, but nor is it something that I would never do a job because of," she says.
Love And Other Drugs has gathered a lot of headlines for showing some casual but refreshingly adult bedroom scenes.
In the 1970s this was no big deal, and in the 1980s female nudity became a reason to pay top dollar to otherwise indistinct talents such as Demi Moore, but with the rise of purposeful hell-no movie queens such as Julia Roberts, bare female A-list flesh has gradually disappeared from the screen.
"Doing nudity really is sort of a part of being an actress. You're aware that at some point it might happen," asserts Hathaway, although she admits that figuring out how they were going to do "this nudity thing" was one of the first conversations she had with their director Edward Zwick.
Famously, she knocked back Katherine Heigl's role in Knocked Up when she discovered Judd Apatow intended an explicit birth scene, even though "it wasn't even my vagina".
This time she wanted clarity, and says it helped that she had been in a similar situation with Gyllenhaal for Brokeback. "We've seen each other partially naked before so it removed any kind of shyness," she says. "It made it very practical. There are no surprises there, nothing to be embarrassed about.
"We really approached it from a very prepared place. We discussed what we wanted to do beforehand, we discussed what we were comfortable with, we traded references from other films in order to establish a sort of index of communication and references that we could all share. So by the time it came time to do those scenes we were very well prepared. And it helped that I knew and trusted Jake."
Hathaway was 21 when they made Brokeback Mountain, and they spent only eight days together as a married couple, compared to the long rehearsal period on Love And Other Drugs which they used to perfect some of the key sequences.
But back in 2005, Hathaway recalls Gyllenhaal shyly asking if he could put his hand on her chest during their love scene. "This time," she says in mock indignation, "he didn't even ask."
Nevertheless, she "fell apart" when the first nude scene came up: "I would have these little panics before we'd shoot the close-ups," she admits.
"And I hate being that kind of needy actor. I love just showing up and doing my job and that didn't happen this time." In the end, to break the tension, Zwick also removed his clothes and climbed into bed with them. "And then he took pictures," she says. And she's not joking – one of them ended up as the poster for the movie, with the small, hairy Zwick digitally removed from shot.
If there were lingering tensions, Hathaway makes sure the strain doesn't show on screen, and the film is respectful of her. The 28-year-old is carefully and discreetly shot, but at the same time there's no Sex And The City coyness here, and certainly no jarring moment where you wonder why on earth someone is wearing a bra and pants during sex.
It has to be said as well that Hathaway has little to fear from the camera's scrutiny. "Cheeky!" she exclaims, grinning, "but I actually found that in the majority of cases the medicine my character takes causes people to lose weight, so that was my jumping off point for how I was going to look in the film.
"My first thought was, I want to be in awesome shape – so it was a relief that the meds make you very skinny but if the medication caused people to gain weight, I would have gained weight and still done the nudity."
Hollywood has always been hard on women and their bodies, and Hathaway admits that in the past she has suffered for other people's art, dieting down to size zero in order to play the long-suffering assistant of fashion plate Meryl Streep in The Devil Wears Prada.
"I was thin for my height. I basically stuck with fruit, vegetables and fish. I wouldn't recommend that," she recalls.
"Emily Blunt and I would clutch at each other and cry because we were so hungry. You see girls who are so effortlessly thin and you do things to be like them but, believe me, I tried everything. My body just doesn't do that, so the best thing I can do is be healthy."
It's significant, too, that every film studio turned down Love And Other Drugs, even after Hathaway and Gyllenhaal signed up. In the end, they took a pay cut. But perhaps it wasn't the nudity that made the studios balk, but the fact Hathaway plays a woman with Stage 1 Parkinson's.
It meant scenes where Hathaway drinks and looks haggard, or at least what passes for haggard when you're Anne Hathaway. For this aspect of the film, she spent time with Parkinson's patients and neurologists and read Michael J Fox's book on living with the disorder.
"I spoke with a few different people with early onset Parkinson's disease. It wasn't just about the physical symptoms, or what the side-effects were from the medication, or what it was like to be on that kind of a schedule, but also the anxiety of being sick. When talking to people, they would always ask, 'Has she accepted her diagnosis yet?' Which led me to believe that there's a whole world of anxiety before you come to that moment.
"Michael J Fox eloquently and gracefully describes his relationship with Parkinson's as evolving to a place where he thinks of it as a gift but it's quite a journey to get there and I think one of the strengths of the film – and one of the things that drew me to my character – is getting to play that."
There's clearly a mental toughness to Hathaway that belies her ingenue appearance, and her own frequent self-deprecations. She started a university degree before films carried her off, and the study element of her job is something she relishes, pulling out every biography possible on Jane Austen for Becoming Jane, and citing influences as diverse as Debbie Harry, Nigella Lawson, Greta Garbo and the works of American minimalist artist Dan Flavin as her influences when playing the White Queen in Tim Burton's Alice in Wonderland.
In order to learn about film, she persuaded a director she admires to let her work as a production assistant so she could study it, and him, from close quarters.
Hathaway dates her interest in acting back to the age of four, when she would tell off friends if they failed to inhabit their roles correctly during games of "let's pretend", and developed full and complicated backstories for her Barbie collection.
"I had quite a few, and they all knew each other in a soap opera kind of way that was super-scandalous," she says. "I don't remember a time when I wasn't acting, although I don't know where I got the arrogance or the focus."
Having stage actress Kate McCauley as a mother probably helped, although McCauley and Hathaway's lawyer father Gerald both held her back from acting professionally until she was 14. Instead, she did the usual school plays, took acting classes and got some modelling jobs before finding instant fame – at least among pre-teen girls – with Disney's Princess Diaries and its sunny sequel.
Age 19, she was charming as the American ugly duckling who discovers she's actually foreign royalty and is sent to Julie Andrews to be polished up from a frizzy-haired schoolgirl. It was Hathaway's natural coltishness that won the role, she claims. During the audition she fell off her chair and charmed the director.
"I'm a messy, rip-your-clothes-up kind of klutz," she claims improbably, pushing an ironed strand of hair from her eyes. "I knock over things. I have bruises in the oddest places."
Ella Enchanted gave her another chance to wear a crown opposite Hugh Dancy but eventually she managed to ditch what she calls the "tiara" roles and graduate to working opposite Meryl Streep, James McAvoy, Helena Bonham Carter and Steve Carell.
Brokeback Mountain was one breakthrough, playing a woman who decides she had found her Mr Right, then hardens when she discovers after marriage that he was her Mr Very Wrong. She was Oscar-nominated when she took the hardboiled persona a stage further in Rachel Getting Married, playing a chainsmoking narcissist who causes havoc at a family wedding in a performance that made few concessions to likeability.
And in The Devil Wears Prada, she was earnest, anxious then graceful as the assistant to Meryl Streep's icy magazine boss.
In real life too, Streep is one of Hathaway's role models, along with Emma Thompson and Kate Winslet.
Breaking out of youth-oriented roles in favour of more adult parts is a tough transition for any young actress hoping to build a long career. But Hathaway says she also had to marshall her own expectations when she first started making movies.
"I used to worry too much, take myself too seriously," she says. "I had to learn that perfectionism slows everything down. All it does is keep you motivated by fear."
But as the roles got bigger, so did the press interest. After reading about tabloids raking through a child star's bins for gossip, her father bought her first paper shredder but Hathaway appeared to be skeleton-free. A devout Christian, she performed good works on behalf of charities, failed to throw diva strops on set and refused to get drawn into catty comparisons with other stars. At a press conference in February, a foreign journalist asked her how she managed to stay so scandal-free "when other actresses like Lindsay Lohan seem to keep forgetting to wear panties".
Hathaway blushed and then laughed, but the journalist did not get his quote. The closest she has come to controversy was when her former boyfriend Raffaello Follieri was arrested on money-laundering and fraud charges.
Follieri was sentenced to four and a half years in jail for swindling investors out of millions by falsely claiming he had Vatican connections that were going to let him buy church property at a discount. Some of Hathaway's jewellery was confiscated as evidence, and later auctioned by US marshals. Now dating actor Adam Shulman, she clams up on her ex and refuses to discuss the matter directly, except to characterise herself as perhaps "too trusting".
Hathaway is, however, game to send up her life, by appearing on the sketch show Saturday Night Live as an all too credulous version of herself, prepared to believe anything she was told. She returned this season to spoof herself again, along with impressions of Katie Holmes, Judy Garland and Kate Middleton.
The Academy Awards organisers caught the performance and sent out an invitation to co-host the Academy Awards next March alongside James Franco. Hathaway will be the first woman to host the show since 1987. "I'm still a little speechless about it, to be honest," she says, "but I have a good feeling about it."
"James was attached and agreed to do it before I had signed up," she explains. "I think the world of him and his taste. So the thought of getting to do it alongside of him was too good of an opportunity." Together Franco and Hathaway's combined ages are less than one of last year's co-hosts, Steve Martin.
Hathaway plans on contacting previous hosts for a little advice – "I have a nice little Rolodex I'm trying to work out" – before hosting the three-hour gongathon. But of course there's always a chance that both she and Franco will be doing double duty as both hosts and nominees. Franco has been generating Oscar talk with Danny Boyle's 127 Hours and Hathaway has been winning plaudits for Love And Other Drugs. She is too smart to overanticipate or jinx anything at this stage, though.
"It's like when you find out that a boy you like has a crush on you too," she allows, "but he hasn't asked you out yet."
Love and Other Drugs is released 26 December
This article was first published in Scotland On Sunday, 19 December, 2010