This is 40, according to Judd Apatow

Producer, writer and director Judd Apatow, described as Hollywood's current king of comedy, hired his entire immediate family for his latest film, This Is 40.

Many, if not most, people would shy away from bringing their kids to work. Apatow isn't necessarily an exception to the rule.

"I think the whole idea of making a movie like this is a little insane," he concedes. "There's a reason why people's families don't star in their movies, there's something risky about it personally."

The 45-year-old has worked alongside his wife, Leslie Mann, and their daughters Maude, 13, and Iris, 8, before, but he admits his work is getting "more personal".

"When you know people intimately you can get to more interesting places. It's much easier to access people's truth when you know them well," he says.

This Is 40 is Apatow's first sequel - a follow-up to the 2007 hit Knocked Up. Paul Rudd and Mann reprise their roles as Pete and Debbie, who were supporting characters in the first film. This time, their marriage takes centre stage as they approach their 40th birthdays.

Debbie is juggling family life with running a business, where one of her employees is suspected of stealing from her. Meanwhile, she is attempting to build bridges with her father, her husband's record label is failing, and there are a number of convincing clashes with her real-life, and on-screen, eldest daughter.

So, was it stressful to play out all of those issues as a family?

"My kids are funny and we all get along," says Apatow. "We're a big gang and we like each other so for them it's just play.

"They don't feel any stress about work because they're not actually trying to be actresses. It's just some amusing anomaly."

Leslie Mann stars alongside her daughters Iris and Maude Apatow

The film was shot down the street from Apatow's home and he admits parts of the story echo his own family life, especially his children's sibling rivalry.

"It really does capture the way they interact and that makes me laugh. They're brutal on each other but it is really funny to watch them spar all day long," he says.

"When they look at each other, you can tell they're really mad at each other and they believe the positions they are stating when they fight in the movie."

But the film has been a form of family therapy.

"Making the movie has made my kids understand each other more and they've gotten along a lot better as a result of having to play out all of these issues on film."

The director says about a third of the scenes are based on experience - but his wife maintains there are big differences between their home life and the story.

"Some of it is inspired by things that we have gone through as a married couple or as parents, but by the time it winds up on film it is something completely different," says Mann.

Paul Rudd and Leslie Mann first starred as Pete and Debbie in Knocked Up

The actress doesn't get a writing credit on the movie, but Apatow says she was deeply involved in the writing.

"I'll tell her my idea for the movie and some scene ideas and then she'll pitch me ideas," he says.

"She's not afraid to be vulnerable. She pushes me to go further."

Apatow's approach is collaborative. He started out writing jokes for Jim Carey and Rosanne Barr and has been involved in developing a string of hits including Bridesmaids and Anchorman.

"My dream was to be a stand up comedian, but at some point I realised that I was better at writing for people than performing myself," he says.

"It took me about 10 years to slowly discover my own sensibility and that's when I started directing."

This is 40 is Apatow's fourth turn at feature film directing, following The 40 Year Old Virgin, Knocked Up and Funny People.

"I didn't consciously think I was having some kind of mid-life meltdown, but I have made two movies with 40 in the title so that must mean something," he laughs.

His own age anxieties are not the only fuel for Apatow's distinct brand of comedy. He has also made a success out of fostering comedy actors and writers, often at the beginning of their careers.

Girls creator Lena Dunham also appears in This is 40

"As a producer, there are comedy actors and actresses that I take to and I want to help them get their visions on the screen.

"So when I meet someone like Lena Dunham [Girls] or Kirsten Wiig [Bridesmaids] it's about trying to understand what they want to do and trying to figure out how to help them get there.

"That's really fun for me, especially when people are doing their first starring roles. They have so much energy and they're so excited. It's a great moment."

Apatow's latest collaboration helped writer, director and star Dunham launch her own successful HBO series, Girls.

Dunham "is very honest," he says. "She wants to talk about her life and how she sees the world. She's not afraid of people's reaction to it, she's not obsessed with likeability.

"She just cares about expressing herself and that inspires me to go all the way, to not hold back and not censor myself."

If This is 40 is an uncensored glimpse into Apatow's mid-life, what should we expect to see as he gets older?

"I am sure I will do my funny version of Amour at some point," he says.

This is 40 is in UK cinemas on 14 February.