At one point during the This Is 40 blooper reel, Judd Apatow's wife Leslie Mann, the film's co-lead, is discussing someone's vagina. "Does yours look like that?" somebody asks. "Not any more," she says, forlornly. "It looks like that... just longer."

The exchange didn't make the final cut. "That's actually the only time during the entire shoot where she said, 'I don't wanna say that,'" laughs Apatow, in London to promote the film. "People ask how I get her to do certain things on camera, and most of them are things she pushed to go further, like how it feels to be in a bar and be hit on when you're 40: how happy it makes you, how much you need it, and how bad you feel if you don't think you're getting as much attention at home. It takes a lot of courage to do that on film, and it becomes an interesting conversation with each other that we share with people by making a movie like this."

Though fictionalised, This Is 40 explores Apatow and Mann's familial dysfunction, mining communication breakdowns for laughs. It's his fourth film as director, and they've become more personal each time, increasingly drawing from his own life; This Is 40 is certainly more reflective than The 40 Year-Old Virgin.

"The more work I do, the more interested I become in personal film-making," he says. "When I first started I just thought of silly premises, big broad comedies, but in my own directing I'm attracted to exploring something I'm going through. At the time I don't even know why I'm doing it. I made Funny People [whose central character has cancer] because a lot of my family had been sick; my mom just died, and I had to watch a lot of people face their own mortality. And there's no way for me to process it other than to write about it. With this, I thought it was the right time to explore what people do to make their relationships work over the long haul, because I've always found that funny. There's something inherently difficult about trying to spin all the plates in your life. And people have connected to it because they see themselves in it."

People have connected with This is 40, but there are also those who have failed to either identify or sympathise with the central couple of Mann and Paul Rudd, whose primary anxiety seems to be that they may have to downsize their fabulous house. "I got a sense of that criticism, that there's a narcissism to these characters," says Apatow. "Some people don't realise that I'm commenting on self-involvement. In a way a lot of these stories are examples of what not to do in a Buddhist pop psychology book. People not living in the moment, being too attached, concerned about money, wanting everything to be perfect, trying to control everything."

'It sounds kind of silly but I think: What if a John Cassavetes movie was really hilarious? … What if a Robert Altman movie had gigantic laughs? Can it be really truthful and really funny?'

Toilet humour: Mrs Apatow, Leslie Mann, and Paul Rudd (seated) in This Is 40. Photograph: Allstar

Apatow says he's recognised similarities in some of the criticism received by Girls, Lena Dunham's much-discussed HBO series. "Some people think Lena is hysterical, and they watch her stumble her way through her 20s," he says. "Other people take great offence to it, as if she doesn't understand that she's commenting on the mistakes that people make, about girls who feel self-entitled and are experimenting in a self-involved way." Apatow is the executive producer of Girls. He also co-writes an episode each season. It's another aspect of his career: his ability and willingness to nurture other comedic talent, getting involved with Girls after seeing Dunham's micro-budget film Tiny Furniture, and helping to fast-track it on HBO. The abundance of empathetic characters in film and TV comedy today is due in no small part to Apatow's successes. Dunham herself recently lauded him for telling truthful, emotional stories while making audiences laugh riotously. "He's created a whole new idea about what comedy can be," she told American GQ.

"My original intention was to merge broad comedies, like the Farrelly brothers or Wedding Crashers, with the human approach that James Brooks and Cameron Crowe take to their work," says Apatow, expanding on the theme. "But lately it's as if I'm trying to merge that work with more independent-spirited film-makers like Robert Altman, John Cassavetes and François Truffaut. It sounds kind of silly to say that, but I think, 'What if a John Cassavetes movie was really hilarious?' There are great moments of humour in them, but what if they were really funny? What if a Robert Altman movie had gigantic laughs in it? Can it be really truthful and really funny? That's what I think James Brooks accomplishes. Terms Of Endearment is about a woman and her failed marriage and people cheating on each other, and cancer, and then her mum dealing with her death, and what's gonna happen to her family... It couldn't sound more dramatic, and it's actually one of the funniest movies I've ever seen. Broadcast News is a love triangle, it's very honest, they don't wind up together, it's tough on these characters, it's not as if any of them are completely admirable, they make all sorts of mistakes. It's a big influence on This Is 40, but also Girls: there's a right guy and she goes for the wrong guy and then she doesn't want any of them. And that's beautiful and truthful and tough, and not an easy pitch. They don't even make movies like that any more. I'm fighting to continue to make movies with that spirit, and it's a fun challenge. Audiences are trained to expect a certain type of movie, but I'm more concerned with doing something different, and most people are thrilled to not get what they always get."

There are profound similarities between Girls and Apatow's other seminal TV show, 1999's Freaks And Geeks, which, despite being mistreated and cancelled by its network at the time, has been hugely influential, its effects continuing to reverberate today. Created by Bridesmaids director Paul Feig and exec produced, co-written and co-directed by Apatow, it tackled the hell out of high school with a grace, wit and subtlety unshared by its contemporaries. In what would become Apatow's hallmark, it was hilarious and touching. It's poetic that the same actress plays the mother of both shows' lead characters. "That might be unconscious," he says. "I just love Becky Ann Baker and felt terrible that I hadn't worked with her in 10 years. But on a deeper level I'm sure it's because I do feel the connection between the shows, and it's a way for me to solidify that. Yeah, they're both interested in truth, most of all. Sometimes they're funny, sometimes they're sad, sometimes they're depressing; they're examinations of time periods, they star people who are not supermodels. Lena Dunham is an adult female version of one of the geeks! These are people who are really smart and might do great things, but struggle to figure out their social issues. That's my favourite thing to explore, people trying to figure out who they are."

Early on, Apatow was shaped by working on Garry Shandling's sitcom The Larry Sanders Show as a writer and consulting producer in the mid-1990s. "Garry blurred the line. Things that happened in his real life would be put into the show, and I thought it was fantastic," he recalls. "I've learned so much about writing from him, and I think Girls is an extension of that approach; Lena's taking her life and fictionalising it. And that's the connection between Girls and This Is 40. We were writing them both at the same time in the same room, trading scripts back and forth. Lena and [executive producer] Jenni Konner were noting me on my drafts and I was noting them on theirs." What sort of things were they suggesting? "They were just helping me sort through all my ideas and talking about structure. Because the structure of This Is 40 is much closer to television. Television is afforded the privilege of rambling around and showing you little moments; it doesn't have to resolve itself, it doesn't have to say, 'They figured it all out, the end.' Things are a mess and they continue to fight on another day."

'What's fun about being around Lena [Dunham] is seeing how far she'll take it … She's not making choices because she's hoping audiences will like her, she's just being brave as a creative person'

Lena Dunham as Hannah Horvath in Girls. Photograph: HBO

We talk about this evolution, different generations picking up from each other, inspiring each other in equal measure. Dunham is taking what Apatow's been doing over the years and creating something new, and he in turn is being influenced by her. "Absolutely, and I felt that when I was on The Larry Sanders Show," he says. "Garry had me around, I think, because he wanted people on his writing staff to push him in other directions; he wanted to be challenged, he wanted that energy around, and that's what's fun about being around Lena. I see what she does and how far she'll take it. When we made the first season of Girls we didn't know if anyone would like it. She's not making choices because she's hoping the audience will like her, she's just being brave as a creative person, and it's great to have people around with that ethic. Everything about Hollywood is about how to get the most possible people to come. But sometimes you get the most people to come because you've done something original. You can reach more people because you're being so confessional and intimate that you make a deeper connection."

Apatow is often referred to as a brand, not just for his distinctive style but his sheer omnipresence on the comedy scene. Yet as well as being the reigning king of dramedy, he continues to be involved with broader comedy, producing outright lol-fests such as Step Brothers and Anchorman. Shooting finally begins on Anchorman 2 next month. "There's no part of them [Will Ferrell and director Adam McKay] that's sweating over the pressure of making a sequel, they just wrote something that makes them laugh," he says. "It's so hilarious and unique, and satirical, I'm excited to be on it."

He's been responsible for countless hits and careers, having produced some of the biggest comedies in recent years, including Bridesmaids and Superbad, and loves nothing more than to nurture people and create opportunities for them, he says. "I like that, because someone – Garry Shandling – did that to me, so I feel like it's natural to do it for other people. And I like being there at the moment of inception. It's much more fun to figure out how to break someone as a comedy star than to do their 35th movie. The amount of effort you get and how much pleasure they take out of the experience is pretty remarkable."

Some of the actors cast by Apatow and Feig for Freaks And Geeks left college to join the show, and he's been employing them ever since. Apatow maintains that it's the best thing he's ever worked on and he is, in a way, still working on it. "It was so heartbreaking when it ended, and when [his follow-up] Undeclared ended," he admits. "But at the time there wasn't much single-camera comedy or dramedy, and there was no place for us. They didn't know how to market it or programme for it. So I stopped working in TV, I was devastated. Almost everything I've done in film has been a reaction to Freaks And Geeks being cancelled, because I loved those characters and those people and I wanted to explore all those ideas.

"You can say Knocked Up is an episode of Freaks And Geeks about Ken Miller [Seth Rogen's character from the show] getting a girl pregnant! Forgetting Sarah Marshall could be Nick Andopolis [Jason Segel's character] broken-hearted. It's a way of pretending the show wasn't cancelled. It was also a way of saying it wasn't fair that the show was cancelled. I just kept going as if me and the writers and directors were still making episodes. Except now they're movies."



This is 40 is on general release in the UK now