IF YOU'VE SEEN the commercials for Judd Apatow's "This is 40," you may have seen Leslie Mann asking permission to touch co-star Megan Fox's famous chest.

It's never a bad idea to feature Fox or her figure in a TV commercial, but in fairness to Apatow, it also happens to be one of the movie's most telling scenes.

"This is 40," as the title implies, is very much a movie about fear of growing old. Mann, Apatow's real-life wife and collaborator, plays a woman turning 40, and who sees in Fox the flickering, fading ghost of bra sizes past.

"Leslie and I both have a lot of emotional issues with time. For me, my family wasn't religious at all. They were atheists. They didn't talk about it much, but the only spiritual advice they ever gave me was: Nobody ever said life was fair," said the director, who stopped in Philadelphia recently to promote the movie.

"When you don't think you're going to heaven, or you don't have some sense of an afterlife, you're in a existential nightmare. You're counting the clock. And I'm counting the clock differently than Ralph Reed" (a conservative political activist).

"This is 40" takes the Paul Rudd and Leslie Mann characters from "Knocked Up" and fast-forwards them to age 40. At this precarious stage in life, she's pretending she's 39, sneaking cigarettes, he's cycling like a maniac to compensate for his two-pack-a-day cupcake habit.

"When you reach your 40s, you've settled into what is your life. You realize: I'm not going to be the ambassador to Algeria. This is it."

For his wife, on screen and in real life, age is linked to fertility.

"You get to be in your late 30s and you get to the point where you say well, I guess we're done having kids. And the kids you do have don't need as much from you," he said. "Then you realize the kids are going to leave one day, and it's just going to be us."

The trick is to make all of this funny - especially when Rudd and Mann are having a pitched battle. Apatow likes to shoot several versions of key scenes, scenes that are developed after much improvisation, discussion, and informal rehearsal.

"I'll shoot jokes, just in case I need to give the audience a break, and then in post [production] I'll decide how many jokes I need. If they're having this fight, and it's a five-minute fight, if I get three laughs in five minutes, I see if that's enough to make this bearable to watch."

Bearable - Apatow wanted the sweet spot between comedy and an honest depiction of marital bickering (he prefers the term negotiation). He hopes people see themselves, and laugh.

"Leslie always talks about how she finds it frustrating that marriages in movies and TV are filled with people being kissy face and caressing each other," Apatow said. "They don't have real problems and they never scream and curse at each other. In her opinion, that makes people feel bad that their lives aren't perfect. And the truth is that marriage is full of wonderful moments, and also a lot of conflict."

And not, in the end, a lot of time to enjoy the moments, and work it all out. Apatow's eldest daughter is already a teenager, soon to be looking for colleges.

"As each phase passes, I'm bummed. That's why I'm reading a lot about Buddhism. I've got to get me some more lives."