Leslie Mann: 'If I don't have a creative outlet, I implode' – interview

It's ironic, given her surname, that Leslie Mann should have gained a reputation as a woman's woman. But over the past few years, Mann has built up a considerable fanbase for her appealingly natural comic presence in films that would otherwise be dominated by frat-boy humour. She was a scene-stealing girlfriend in The Cable Guy with Jim Carrey (1996), appeared alongside Adam Sandler in Big Daddy (1999) and famously threw up over Steve Carell in The 40-Year-Old Virgin (2005). Her big breakthrough came in the 2007 hit Knocked Up, in which she played Debbie, the highly strung older sister of a twentysomething TV presenter who finds herself pregnant after a one-night stand.

Mann is currently starring in This is 40, "the sort-of sequel" to Knocked Up, in which she reprises her role as Debbie, whom we now revisit on the brink of middle age. Both films were written and directed by Mann's real-life husband, Judd Apatow, and feature performances by the couple's daughters, Maude, 14, and Iris, 10.

"I loved being able to take the story on," Mann says when we meet in a hotel suite in central London. "With Knocked Up, it was like a little window into this couple's life and it was fun to explain where they're both coming from now.

"It's really important to me to make sure that the female voice is represented as well as the male and I think Judd did a good job of balance so that you don't sympathise with the guy more than the woman. You can be crying one moment and laughing out loud the next... which is my favourite type of movie."

Mann is midway through a long day of promotional interviews and arrives holding a crumpled tissue, having first ordered hot water, honey and lemon to stave off a cold that has been plaguing her since stepping off the long-haul flight from her home in Los Angeles. The paraphernalia of illness makes it impossible not to inquire how she's feeling.

"Thank you," she says, with rheumy eyes. "You're the only one to have asked."

She appears extremely delicate: thin and pale, wearing uncomfortable-looking heels and a tired smile. Yet in other respects, she appears hardly to have aged in the years since Knocked Up – her forehead has the unlined sheen of a well-polished car bonnet.

But in This is 40, Mann puts in a convincing performance as a woman struggling to come to terms with the passing of the years. She stars opposite Paul Rudd, who once again takes on the role of Pete, her well-meaning but emotionally avoidant husband. The couple are ostensibly still in love while perpetually on the edge of explosive mutual irritation. In one extremely funny scene, they have a frank conversation about how they both imagined killing the other. "I'd poison your cupcakes," Debbie tells Pete, "and just put enough in to slowly weaken you. I'd enjoy our last few months together because you'd be so weak and sweet and I could take care of you."

Mann herself turned 40 last March. Did she have a problem with it?

"Not yet," she says. "Thirty was harder, I think, because I had almost completely given myself over – it was like I didn't exist. I was a mother, a slave to my kids and I had tremendous guilt about being away from them. I just lost myself completely so I kind of had my nervous breakdown at 30 and then, at 40, I just feel I understand things so much more. My kids need me that little bit less. I have time to do some of the things I want to do. I'm being allowed a creative outlet and that's a big one for me. If I don't have that, I implode."

Part of the reason she has found the past few years so satisfying creatively is, of course, to do with her husband's ascent. Apatow has been widely credited with reinventing mainstream American comedy: dragging it from gross-out scripts and twee romcoms into a new era of ensemble acting and smarter scriptwriting, producing films that are simultaneously more subtle and much funnier.

Mann has been an integral part of this trajectory. "I don't know anybody who is as strong an actress," Apatow has said in the past, "who is willing to leave her guts on the floor and also riotously funny. It's a rare combination."

The couple met at a script-reading when Mann was auditioning for The Cable Guy, which Apatow co-wrote and produced. Apatow has since described it as "love at first sight". Was it the same for her?

"No, I don't remember him from that moment," she replies. "Judd was reading Jim Carrey's part and at that time I had a little bit of a crush on Ben Stiller [the film's director] so Judd didn't get a look-in."

I say that I always imagined Ben Stiller would be very short. Mann considers this carefully.

"He's not that short," she says finally. "Probably around 5ft 9in. But he's stout."

Eventually Apatow managed to persuade the object of his affections not only to marry him and bear his children but also to become a willing collaborator in his unique brand of improvised comedy. In The 40-Year-Old Virgin, it was Mann who insisted that the key scene in which she drunkenly flirts with Steve Carell's sexual innocent should end with her vomiting in his face.

"Yeah," she nods. "I made the vomit myself. It was a mixture of liquid yoghurt and some sort of chunky breakfast cereal. That was fun."

But in This is 40, the collaboration goes deeper than pranks and pratfalls. Underlying the film's humour is a poignant and truthful analysis of the difficulties inherent in sustaining a long-term marriage. What was it like playing a wife on the verge of a marital breakdown, acting from a script written by your own husband who is also directing you from behind the camera?

Mann rolls her eyes. It's a question she's clearly been asked a hundred times before. "Sometimes we would have arguments through the characters – things we were afraid to say to each other," she says. "But by the time it winds up in the film, it isn't at all what we're like… I don't know, it doesn't feel to me at all autobiographical."

She "loved" working with her daughters. "They're beginning to figure out how it all works. They're confident enough now to pitch jokes and contribute to the creative process… But they're not allowed to act in anything else until they're older," she adds quickly. "Child actors don't have great track records."

And yet Mann herself started acting as a teenager. She was raised by her mother, an estate agent in Newport Beach, California, who married three times. Mann's father was absent and she describes herself as an adolescent as "pent-up and shy". When she started auditioning for roles aged 18, she found acting was "a safe place to express myself". She later studied with the Groundlings, the improvisation troupe in LA whose alumni include Will Ferrell and Maya Rudolph.

"Growing up, I wasn't as comfortable expressing myself as I am now and I think that's why I chose acting because it's acceptable to have your feelings," Mann explains. "It's a place that they want you to feel. Whereas in life, growing up, it was 'Be quiet!' and 'Keep it to yourself.'"

She looks at me, her smile not quite reaching her eyes, and it strikes me that there is something terribly sad about this admission. Has her relationship with Apatow – professional and personal – given her the confidence she once lacked?

"Probably it has helped," she says. "He's very supportive of me and always has been. He's always been a great sounding board for me and a great voice of logic. He's grounded me in a lot of ways. I'm very emotional and he's more intellectual, so we balance each other out for the most part."

I know it's unfair to expect actors to be as entertaining in person as they are in films but, for some reason, I had imagined Mann would be a little less earnest. (Later, she tells me she's reading a book on meditation because "it could be really helpful in my life".) Her onscreen persona is so likable – charismatic but not intimidating, witty but not brittle – that I'd assumed it would translate into real life. Perhaps it's because she's ill. Perhaps it's that she's sick of being asked questions about her marriage. Whatever the reason, Mann seems a little downbeat.

In an attempt to lighten the mood, I ask her whether it has become easier for female performers in recent years with the success of writers and actors such as Tina Fey, Lena Dunham and Kristen Wiig.

Mann nods. "Yes, it seems completely different now. It felt like, when I was 22 or whatever, people would always say 'Your career is over at a certain age' so I just kind of believed that. But it seems that's an outdated way of thinking. That's not reality any more. It feels like the doors are wide open [for women]. If you want to be in a movie, you sit down and write a movie. You can do anything… Before, that wasn't available to us. You've got such talented women who are writing such great material… Lena, I mean, Lena… she's so incredible."

At this point, Mann visibly wells up. "She's such an inspiration," she continues, her voice breaking. "Her voice is so powerful. It's not about what she looks like physically. She's so brave." The tears spill out. Mann starts to cry, mopping ineffectually at her face with the tissue. "I'm sorry. I don't even know why I'm crying… I'm just super-happy for my kids growing up at this time. It feels like a big change."

The interview never really recovers from this unexpected flurry of weeping. When I ask her which actresses she most admires, she lists Ruth Gordon and Shirley MacLaine. "My favourite movie is Terms of Endearment," she says, naming what is possibly the most tear-jerking film of all time, in which MacLaine stars as a mother nursing her daughter through terminal cancer.

Given her admiration for Dunham, has she ever thought of writing herself? "Not right now," she says briskly. "Maybe when my kids are older, when I have more time."

The PR pops her head round the door to signal the interview is over. Mann pushes herself off the sofa and smooths down her minuscule dress, as if mentally preparing for the next onslaught. She says a polite goodbye and then leaves, still clutching a balled-up tissue in one hand.