Interview: Maggie Gyllenhaal, actress

'WHAT does it matter? What do you mean by genuine?" I've annoyed Maggie Gyllenhaal, but I didn't mean to. We were talking about celebrities who get involved with "causes" and I made what I thought was the innocuous point that it can be hard to swallow when there's a suspicion that it's a publicity stunt rather than a real commitment.

"OK, let's take an example that's not at all a controversial topic," Gyllenhaal says, and I feel like I'm about to be seriously set right. "Let's say an actor is telling people to go and be an environmentalist or save the whale. Let's say they're not totally perfect about it." By that I think she means they don't recycle their every yoghurt pot or knit their own shoes.

"They're doing their best," she says, her voice rising. "You know, they wear the T-shirt, they get in front of the camera and say 'pay attention to this'. Isn't that OK? Isn't that still good? It's better, of course, if you really do make it a part of your life, but isn't it better to do something helpful, to do as much as you can?"

Gyllenhaal laughs, but it's clear she's irked. And it's fair enough. She is an actor who has worn her opinions as openly as the Miu Miu handbags of which she's fond. With her husband, Peter Sarsgaard, she's one half of the most credible acting pairings around. In her own right, Gyllenhaal is articulate and unabashedly smart. And the reason I didn't really mean to knock that, which is what she thinks I've done, is that in Hollywood terms, she's refreshingly, almost shockingly, unusual.

"I know there are a lot of people who think actors shouldn't be political," she says, "but I'm a political person and there's no way that I can just decide not to be; that doesn't make any sense to me."

This is not the only way in which Gyllenhaal differs from regulation Hollywood. She doesn't look like an identikit starlet; the slopy eyes and the lopsided smile ensure that. Her career's not average either. In the beginning she was spanked by James Spader in Secretary (2003). Then there were plaudits for her gritty portrayal of an ex-convict in Sherrybaby (2007). And even when she landed a part in a full-blown blockbuster, Christopher Nolan's The Dark Knight (2008), she berated Bruce Wayne for his shallow hedonism, bringing a depth to Rachel Dawes that the original incumbent, Katie Holmes, could only dream about.

Gyllenhaal is a formidable actor. As LN, in Sam Mendes' Away We Go, she flexes her comedic muscles, putting in a funny turn and bagging the best lines as an eccentric earth mother and academic. "My brother (Jake Gyllenhaal] and my husband have worked with Sam, so we know him," she says, pausing as she drags on her cigarette. "I really liked the script. I got a sense that I could really play and have a good time."

Mendes' whimsical comedy drama follows Burt (John Krasinski) and Verona (Maya Rudolph) on an idiosyncratic roadtrip to decide where to settle and start their family. Gyllenhaal's character, LN, is a relation of Burt, and from breastfeeding two babies at once, including a boy who looks old enough to poach his own eggs, and proselytising on the benefits of the entire family sleeping together in an enormous bed, it does look like she had fun.

There has been criticism that Vendela Vida and Dave Eggers' screenplay tips into cruelty, with characters who are so caricatured the audience can't empathise with them, but Gyllenhaal is having none of it.

"I do know a lot of people like LN," she says. "Not exactly, I mean she's definitely a send-up, but a bit like that. And I definitely have a bit of that in me. Before I was a mom (Gyllenhaal and Sarsgaard's daughter, Ramona, is two] I'd be like: everything's going to be organic and I'm going to use cloth nappies and I'll wash them myself. And then you're a mom and you're like: wait a minute, I'm on an airplane with my 14-month-old and there's nothing to eat but potato chips and she's going to eat them because what else am I going to do?

"I think the film is compassionate. I thought this is a movie about people who are not perfect, they're dealing with the things I'm dealing with – making a family, finding a home. They're struggling in so many ways that I understand that it made me laugh out of compassion. But look, there is something cruel about comedy. That's part of it."

Throughout her career Gyllenhaal has confounded expectations by flitting between indie projects and mainstream blockbusters. In the middle she's also done stints on stage, most recently in an off-Broadway production of Uncle Vanya, directed by "legend" Austin Pendleton, in which she starred with Sarsgaard.

"It was the best experience I've ever had as an actor," she says. "I loved it. The play was barely staged, we were really different every night. That's how I like to work, I don't really like to have things planned."

The experience has given her the urge to do more Chekhov. Three Sisters would be next, ideally, she says. But what was it like working with Sarsgaard?

"I always knew he was a brilliant actor," she says. "We could have a great marriage and not be able to work together, but it just came so easily to us and it was such a pleasure. I've never been more inspired and excited."

Gyllenhaal says she feels "fluid" about the projects she chooses to work on, happy to mix big and small as suits her. But she's pragmatic about what drives her and what commitment it takes to get involved with a film which might have no money attached, meaning that she will have to use her name to get backing and then play her part in getting it sold. It takes three years, she says, and it's not a commitment that she's always willing to make.

"Sometimes I'll read a script that has no money and it's a tiny independent, and I'll think if this had its money and it was being made by a studio I'd do it for six months," she says. "But I know I can't carry it on my back, getting funding and then selling it once it's made. I'm just not that interested. You have to know what you're taking on."

For an actor who talks about inspiration and experimentation, Gyllenhaal is clear-eyed about the business side of making movies. In fact, she likes it. She knows that mainstream movies give her the clout to make the other kinds of projects she's interested in happen.

The only topic that she seems equivocal about is the other cog in the movie machine: press intrusion. She might feel that it's an actor's responsibility to know about "what's going on in the world". ("If I care about the state of the world then I should say pay attention to who the president is and what his policies are, pay attention to Iran and what's happening in Gaza," she says.) She doesn't, on the other hand, feel that she has to be available constantly for public consumption.

"It was really bad when my daughter was first born," she says. "We were living in the West Village, in Manhattan. But since we've gone to Brooklyn (Gyllenhaal and Sarsgaard live in a Brownstone there] it's like we're not interesting enough for them to cross the bridge for. It's fine by us."

The couple choose projects that allow them to split the care of their daughter, and have spent much of the past few years working in London. For the past four months Gyllenhaal has been shooting the Nanny McPhee sequel. The family got comfortable, she says, living in the same area, putting Ramona into a nursery, but Gyllenhaal can't wait to get home.

"It's not really New York," she says. "I love New York and I've lived there for a long time but it's really more my house and friends. All my friends have little kids so it's hard for them to come and visit."

However much she's craving domesticity, though, I suspect it won't be long until she's working again. "People think differently about acting," she says. "Some think it's an artistic form and for others it's much more a business. Honestly I think it's a mixture of the two and I think that's fine."

But none the less, she's clear what side she comes down on. "I do think it's an art form," she says. "I feel like an artist."

• Away We Go is on general release from 18 September