IT'S A muggy, overcast Tuesday afternoon in London and a storm is brewing. Not a real storm, but one of those media ones you see on the news any time Amy Winehouse leaves her front door.

The setting is the Dorchester Hotel, and I'm waiting to interview Maggie Gyllenhaal about her role in the new Batman film, The Dark Knight. This would be no big deal in itself, except that a couple of hours earlier, reports had started filtering through that her co-star, Christian Bale, was taken into police custody for questioning over an alleged assault charge filed against him by his mother and sister.

Bale will later be released without charge, but the "Batman Arrested" headlines are being prepared, and camera crews and paparazzi are swarming all over the hotel's Park Lane concourse.

It's probably fair to say this was not the kind of publicity the film's arrival in Britain was supposed to generate. Indeed, at the press conference in the Dorchester the previous day, most of the attention was focused on the film's critical success and its record-breaking opening weekend in the US (just north of $158 million – about £80 million). There were no hints that anything more was afoot. Even the proverbial elephant in the room – the death earlier this year of Heath Ledger from an accidental overdose of prescription drugs – didn't stir up much controversy, with journalists respectfully adhering to the publicist's requests to limit questions to his performance as the Joker (it's as good as you've heard, by the way) rather than the sad circumstances surrounding the 28-year-old actor's demise.

By the time I'm finally taken in to meet Gyllenhaal, she seems to have grown slightly weary of the whole media circus. That's perhaps understandable: if the interview clips that will later air on the evening's television news bulletins are anything to go by, she has just spent much of the afternoon deflecting questions about her suddenly absent leading man.

Mostly, though, Gyllenhaal is tired simply because of the sheer scale of the publicity machine that she's been a part of for the past few weeks. She may have been in the public eye since her early 20s – when her starring role in the provocative indie hit Secretary put her firmly on the Hollywood radar (along with her status as Jake Gyllenhaal's older sister) – but the 30-year-old actress has never before been part of anything quite so… huge.

"I've barely been able to take it in," she says, sounding a little dazed. "It's just been talk show to talk show, to press junket to flying to London to… I mean, in a way, despite being in Leicester Square last night and despite the massive turnout, I could barely feel it because I've been going and going and going."

The fact that she's shouldering such a large proportion of The Dark Knight's promotional duties reflects her importance to the film's plot. She plays Rachel Dawes, Gotham City's fearless and intensely moral assistant district attorney – who also just happens to be Bruce Wayne/Batman's childhood friend and sort-of ex-girlfriend.

The last time we saw the character was in 2005's Batman Begins, played by Katie Holmes. But thanks to one of those silver-lining scheduling conflicts, it is Gyllenhaal we see up on screen in The Dark Knight, turning not only Batman's head but also the chiselled features of Aaron Eckhart, who plays Rachel's boss, Harvey Dent, the idealistic district attorney determined to clean up Gotham City without the need for masks or rubber suits. It is actually Dent's story (fans of the comic will already know about its tragic dimensions) that provides the film with its spine, and Rachel is integral to it. Torn between her feelings for Wayne and the more tangible force for good that she sees in Dent, she's the lynchpin preventing both men's worlds from descending into chaos.

It's a meaty part, though Gyllenhaal had her reservations about accepting it: "I really didn't want to do it, not until I was sure that Chris (Nolan, co-writer and director] wanted Rachel to be a complicated, fully realised woman who cared as much as the men around her did about making the world she lived in a better place," she says.

"In fact, I wasn't reading scripts at the time. I had a three-month-old daughter (with her fiancé, the actor Peter Sarsgaard] and I wasn't looking for work at all. But when we met, (Nolan] handed me a script and said 'Rachel's not finished yet' and, as we talked, I really felt like he wanted a collaborator, someone who would help him develop who she would be."

Nolan also needed someone who wouldn't crumble when confronted by the Joker. In one of the film's most intense scenes, he gatecrashes a party and threatens to slice Rachel's face open. It's a terrifying moment, but it's also oddly exhilarating. Gyllenhaal puts that down to Heath Ledger: "I think it's very unusual to be totally free in the way that Heath was in this movie. There were no wrong choices and working around that is very contagious, it bleeds into everything, which is why I think you can see that even Rachel is kind of thrilled and exhilarated to be around the Joker. She feels that something really life-changing is about to happen."

Will The Dark Knight's success prove life-changing for Gyllenhaal? It has already changed her attitude to film-making, encouraging her to lose the punk-rock attitude that convinced her it didn't matter how many people watched her films ("Why make movies for the ten people who feel exactly the way I do about the world?").

It will probably also give her a little more clout with the studios. "I hope that happens," says Gyllenhaal, "because right now I feel like there are only two women whose names can help finance a movie."

Just don't expect her to get carried away by success. Having grown up with parents in the business – her dad is a director, her mum an Oscar-nominated screenwriter – she understands how ephemeral it is. "I think just seeing my parents go up and down in terms of their success really helped me, because I see how little it matters, ultimately." And with that she's whirled out the room, breathing a sigh of relief as she goes.