IT is a rare actor who does not worry about on-screen chemistry, that hard to define but obvious-when-it-is-absent, commodity.
Leslie Mann didn't have to ponder it for a second when it came to shooting her new comedy, This is 40.
With her real daughters playing her screen daughters, her husband, Judd Apatow, writing and directing, and a long-time family friend, Paul Rudd, playing her husband, there was more chemistry on the This is 40 set than in the labs of GlaxoSmithKline.
Though she had worked with her daughters Maude, 14, and Iris, 9, on Knocked Up, the movie's "sort of" prequel, Mann could see a transformation in them this time around.
"I noticed on this movie that they both wanted to be a part of the creative process, whereas before they were too little and didn't really care. Iris was pitching jokes and Maude cared more about how she was doing as an actress.
"I was impressed by them, by their confidence, and their not holding back in front of adults. Sometimes children will get shy around adults and not want to speak up or express themselves. They don't have an issue with that, which I like."
In Apatow's 2007 Knocked Up, Pete and Debbie were the averagely unhappy married couple who illustrated how tough it could be to keep a relationship going. Fast forward to 2013's This is 40 and Pete and Debbie are approaching mid-life marital meltdown. Described by its makers as an "unfiltered" comedic look at life inside an American family, it is scorchingly honest at times, with Mann as the mother trying to hold everything together, generating most of the heat.
Given the writerly tendency to draw from life, one has to wonder how much is based on the Apatows' experiences.
"It feels like some things are inspired by things we've experienced in our marriage or in our lives, but by the time it winds up on film it's a completely different animal. It's not my life at all," says Mann, who turned 40 in March last year and has been married to Apatow for 16 years. She didn't look at the movie and think "that's my life up there", but anyone who has been in a relationship for a long time will be able to relate to it, she reckons.
For Apatow, as is increasingly the case with his writing, This is 40 was a chance to take a more considered look at life. Funny People, his previous film, was a shrewd, moving, critically lauded comedy-drama about a comedian (played by Adam Sandler) facing the toughest gig of all: his failing health.
"Lately, I have thought a lot about where I am as a 44-year-old man," says Apatow. "I don't know if it is a mid-life crisis or a simple taking stock, but I definitely have been thinking about how it is going. You get to a certain age and you realise that this is your life.
"You will not run hurdles at the Olympics or live on a mountain in Switzerland. This is my wife and my family. This is my job and the rate at which I am going bald. Then you decide how you feel about it. I am generally thrilled with my life, but at the same time Leslie and I often wonder why certain aspects of our life and relationship are not easier. This movie explores some of those questions."
While Apatow, whose credits run from The 40 Year Old Virgin to HBO's Girls, has been busy making himself Hollywood's king of comedy, Mann has been creating a realm of her own. Her breakthrough hit was The Cable Guy with Jim Carrey (that was also where she met Apatow). She worked with Carrey again, and Ewan McGregor, in I Love You Phillip Morris, voiced an animated character in the Oscar-nominated ParaNorman, and starred in 17 Again with Zac Efron. She will next be seen in The Bling Ring, a comedy drama directed by Sofia Coppola.
She didn't set out to leave such a gap between first meeting Apatow in 1996's Cable Guy and working with him again on 2005's 40 Year Old Virgin. That was just how life worked out: their first daughter came along within the first two years of marriage, then the second five years later, and work fitted around them.
"I had so much guilt around leaving my kids so I didn't. I stayed home with my kids and took time off. I did little things here and there but I pretty much was a mom, and still am. That's why we like doing this, because we can all stay together."
As for husband and wife working together, it's a partnership, says Mann. Yes, she is his wife and stars in some of his films. "But you can also say that Judd [and I] help each other. He's the writer but we collaborated on it. It's not like I'm sitting back waiting to get a job, I'm very much a part of the process."
Mann, it is true, does not seem the type of person to linger in anyone's shadow. Though struggling with a cold, she laughs a lot. But there are places she won't go, lines that are not to be crossed, as when I ask if her views on marriage were influenced by her mother's experiences (she was wed three times). "I don't want to talk about that." Ditto her dad, though she was quoted in a magazine interview last year as saying she had "zero relationship" with him.
Born in San Francisco in 1972, Mann learned her trade at an acting studio and honed her skills in a comedy improv company. She would be happy if her daughters wanted to go into the business, with one crucial proviso.
"I think it's important that they are able to create their own material, that they have ideas and that they aren't reliant on other people to give them jobs. That's where actors get into trouble, it becomes like a desperate thing. Whereas if you can come up with the ideas then you're good."
In the film, Mann's character regards her 40th birthday like the world's greatest party pooper. "I don't want to shop at old ladies' stores," wails Debbie, insisting, despite the evidence, that she is 38 really. How does 40 feel for Mann?
Good, she says, and not at all like she imagined. "I thought when I turned 40 I would be like really old and like a grandma. But now that I'm 40 I kind of don't feel any different in my head. I feel healthier than I've ever been and more confident and comfortable in my own skin. It's way better being 40 than it is 20 or 30. Way better."
She had another young co-star recently in The Bling Ring, Emma Watson, of Harry Potter fame.
"Adorable," she says. As was working with Coppola. "Sofia was the exact opposite of working with Judd. Judd is very intense and demanding, Sofia is so mellow, calm and soothing. Heavenly."
Enough about other people's directorial styles, when is she going to direct her own movie?
"Not any time soon. I have to be a mother right now – and get over my flu."
This is 40 opens on February 14