In “This Is 40,” Judd Apatow’s “Knocked Up” spinoff due Dec. 21, Pete and Debbie take center stage. The film — Apatow’s first directing effort since 2009′s “Funny People” — picks up with the family a few years after “Knocked Up.” Pete’s music label is struggling and Debbie’s vanity business, a dress shop staffed by the comely Desi (Megan Fox) and the awkward Jodi (Charlyne Yi), needs to help cover the household budget.

“It’s the kind of stuff about marriage that you don’t get to see in movies,” said Mann, Apatow’s real-life wife. (The couple’s daughters, Maude and Iris Apatow, now 14 and 9, reprise their “Knocked Up” roles as Paul and Debbie’s children.)

“It’s about getting older. What the hell happened? Am I making all the right choices? Is this where I’m supposed to be? Is this it? All of those dreams that you had when you were young, all of the possibilities, when you lock into marriage and kids, do the possibilities dwindle or go away?”

In keeping with the rest of Apatow’s oeuvre, “This Is 40″ relies heavily on improvised comedy. In a Pacific Palisades dress shop last August, on day 40 of the production, Apatow and Mann’s rapport as director and star and husband and wife was on full display. Apatow called out one-liners from the director’s chair as Mann and Yi riffed a scene about Desi’s assets as a saleswoman.

“She’s way hotter than me. It’s like having Babe Ruth working at your store,” Apatow suggested to Mann.

“She’s hotter than me,” Mann slightly modified the line, with an arched eyebrow. “It’s like having Babe Ruth working at your store.”

“Judd’s behind the monitors listening and watching and writing as we go,” Mann said later describing her husband’s creative process. “You have to totally surrender and trust that he will catch you and support you.”

Paul and Debbie’s travails, Mann said, are partly are based on their 14-year marriage. “The good moments are me and Judd.”

The bad ones?

“Oh, those are all Paul Rudd and his wife.”