LOS ANGELES When Universal releases “This Is 40” on Dec. 21, it will be the most personal film Judd Apatow has made.

It is not only a movie that steers away from the wild or unsettled single men of his earlier efforts (“The 40-Year-Old Virgin”), but one he has created in collaboration with his wife, Leslie Mann, who stars in it with their daughters.

If those are not sufficient indications of Apatow’s investment in the project, there is also the sting he would feel if audiences rejected the film.

“There’s nothing worse than spilling your guts and having people hate it,” he said, chuckling. “ ‘Oh, that’s your world? Wow, I don’t like the movie or you.’ ”

As he explained his thinking behind “This Is 40,” speaking in September at the West Los Angeles office his employees call the Apatower, he said: “Here’s my family, here’s my marriage, here’s my job. Are we happy with where we’re at? Can we make it better?”

The comedy in that, he said, arises “when you try really hard to control it all, and that always comes back and smacks you in the face.”

Despite his tendency to personalize the plot, Apatow says the movie is not really about him and the three-quarters of his nuclear family who appear in it.

In “This Is 40,” Apatow revisits Pete and Debbie (played by Paul Rudd and Mann), the middle-aged parents introduced in “Knocked Up,” as they are barraged with common predicaments: Is Pete’s business failing? Can Debbie be both friend and boss to her employees? Can they raise their children (played by Maude and Iris Apatow) and navigate relationships with their own parents? Is this a hemorrhoid?

Apatow said this intimate specificity – inspired by reality, if not lifted from it – was necessary to connect with the wide audience he seeks.

“People never walk out of the movie and think it’s about us,” Apatow said. “They always think it’s about them.”

Since the births of their daughters, Maude, now 14, and Iris, 10, Apatow said, he sought to convince Mann there was a memorable comedy in their prenatal adventures.

“All sorts of crazy, terrifying, hilarious things were happening,” said Apatow, 44. “I would say, ‘You think we should make a movie about this?’ And I would pray she would say yes.”

In the presence of their children Apatow and Mann, 40, are doting, boo-boo-kissing parents. When it is just the two of them (and a reporter), the dynamic is different: they joke around and tease each other affectionately, but also hang expectantly on each other’s sentences.

With her husband seated next to her, Mann said she was concerned other romantic comedies did not depict domestic squabbles and their aftermaths as she was used to them.

So Apatow created a story for the Pete and Debbie characters that would show what his understanding of marriage looked like while giving him and Mann a creative place to work out innermost feelings.

As his screenplay developed around these conversations, it incorporated experiences he and Mann have had – say, his sneaking off to the bathroom to play games on his iPad – and comic exaggerations of true circumstances; other personal issues were omitted.

“I have vague memories of not wanting to talk about certain things,” Mann said to Apatow, “like your things.”

He replied, “Well, don’t talk about them here.”

Daughters as actress

Maude and Iris Apatow too are playing expanded versions of their “Knocked Up” roles – not just wisecracking moppets this time but characters who are integral to the plot.

Apatow said he shoots with his daughters only during summer and they are not encouraged to act in other people’s films. He said he felt they were ready for some of these challenges and it was important they received this exposure to the family business.

“I’ve tried to explain to them why we do it,” he said. “This is what creative people do. They share their lives, they let other people see that they feel the same things as them – that we’re all in this together.”

Maude Apatow, a blogger used to shrugging off criticism on Twitter, said in response to email questions that she was glad to be included in “This Is 40.” “I would have felt bad if I was replaced by some other kid who looks a little like me,” she wrote.

In some scenes – like a fight with her on-screen parents, after her character has been forbidden to use Wi-Fi – she said it was hard to keep her authentic emotions out of the movie.

“Sometimes I am not acting,” she wrote. “I forget we are acting and I just get irritated for real. Who takes away the Wi-Fi?”

On the other hand, she said, she was proud of her work in a scene in which she had to lose her temper, cry and curse at Mann. “My mom taught me how to commit to the scene, which helped me a lot,” Maude wrote. “Nobody commits harder than my mom.”

Mann said Maude does not use obscenities when they argue but praised her daughter’s performance in that scene. “It was really the moment where she became a little actress,” Mann said.

In the way that Apatow cast the people closest to him in his own movies, Albert Brooks, who plays Paul Rudd’s ne’er-do-well father, compared him to filmmakers Woody Allen and John Cassavetes, with one crucial difference.

“Judd is making big commercial comedies,” Brooks said. “He’s trying to make a movie everybody sees.” Cassavetes made low-budget independent features.

Real people, situations

Apatow said he was taking cues from former bosses and idols Garry Shandling, who put forth a fictionalized version of his professional life on “The Larry Sanders Show,” and James L. Brooks, who focused on ordinary people with ordinary problems (“Terms of Endearment” ).

If one includes his beloved if short-lived TV shows “Freaks and Geeks” and “Undeclared,” Apatow said, he has looked at American lives in nearly every phase of development, from high school onward.

For the lead characters in “This Is 40,” “a sympathetic show of their lives and issues is something that I think needs to be made,” Apatow said. “Every aspect of life is difficult for people. It’s all a little too much. We’re all deeply overwhelmed by it.”