Judd Apatow on Casting His Daughters and Their Dramatic Onscreen Arguments (Video)

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The midlife crises in This Is 40 are meant to ring true, combed from the lives of writer-director Judd Apatow and wife/star Leslie Mann, but it's the performances from their children that might end up being the most realistic -- because they come from a very real place.

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Apatow cast his and Mann's two daughters, Maude (14) and Iris (8) as the children of Pete (Paul Rudd) and Debbie (Mann) in the upcoming film, returning them to the expanded versions of the parts they played in 2007's Knocked Up. In a new longform interview with Conan O'Brien, Apatow talks about how he coaxes his daughters into giving convincing performances. It's not exactly method acting, but it could be considered along those lines.

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"They're not scared that cameras are around," he says. "They get sucked into what their real problems are with each other. Like, they're so annoyed with each other that they're not distracted by the fact that they're shooting a movie and it needs to go well, they're more concerned about whatever argument I'm trying to make them have on camera.

"So if I say, 'OK, Maude, tell her she can't watch Lost, she's too young,' Maude wants her to not watch Lost. She's mad. She's like, 'I'm 14 years old, I'm allowed to watch Lost, and you're 8, you have to wait six years, and if you see it before six years, I got screwed,' " Apatow continues. "And so she's mad, and Iris really doesn't get why she can't watch Lost … and then they just get sucked in, and they forget [they're filming]."

He does, however, feel concerned about thrusting his children into the spotlight. In fact, he has cut out Maude from several films because of that worry -- which has come to annoy his younger daughter. Show business.

The segment on their children starts about 17 minutes in.