'This Is 40': Why Katherine Heigl, Seth Rogen Don't Appear in the 'Knocked Up' Sequel (Video)

With Paul Rudd and Leslie Mann in the lead, This Is 40 also features plenty of notable talent in supporting and cameo roles, including John Lithgow, Megan Fox, Albert Brooks, Graham Parker, Billie Joe Armstrong, Michael Ian Black, Lena Dunham, Ryan Adams, Annie Mumolo, Chris O'Dowd and a very memorable Melissa McCarthy.

our editor recommends

Leslie Mann on the Perks (and Drawbacks) of Acting Opposite Her Daughters in 'This Is 40' (Video)

In Theaters This Week: Reviews of 'Jack Reacher,' 'This Is 40,' 'Zero Dark Thirty' and More

This Is 40: Film Review

Billed as a "sort-of sequel" to 2007's Knocked Up, the film brings back Jason Segel and Charlyne Yi to reprise their roles, but Debbie's (Mann) sister Alison (Katherine Heigl) and her unlikely beau Ben (Seth Rogen) -- who were at the heart of Knocked Up -- are never mentioned in the new comedy.

PHOTOS: Six Degrees of Judd Apatow

Speaking with The Hollywood Reporter, writer and director Judd Apatow explains their absence and notes that another sequel, focused on those characters, could come in the future.

"I thought if the audience saw the cut and said, 'We must know where they are and what they're doing' [we would include them]," says Apatow. "But to me, this was so about Pete and Debbie that if you pass through two incredibly hilarious, charismatic people, the audience would say, 'Well, what else are they doing?'"

"I mean, it's not like you could put them in for a minute," he adds. "You'd have to really give them a big hunk of the movie, and I thought, 'Well that's a movie for another day. I want to kind of stay focused on this story'"

PHOTOS: This Is Judd Apatow: The King of Comedy's Life in Pictures

As for Segel and Yi, the only crossovers outside Pete and Debbie's immediate storyline, Apatow explains: "It seemed like it made sense that Jason Segel, who had a crush on Debbie, would still be hanging around…And Charlyne Yi, who was kind of the stoned girlfriend of Martin Starr, I thought it would be odd to choose her to be the one person that you would see from that world."

But it's McCarthy who will leave audiences reeling, stealing the show as a school mom who takes aim at Pete and Debbie's parenting skills.

"She came in and she read for a couple of parts in the movie and that just made me laugh, that attitude," Apatow says of McCarthy's angry mother. "The person who gets madder and madder, and then can't control themselves. We shot all her stuff in two days, and every second she was ridiculously funny."

While her part is brief, Apatow assures us that there will be plenty more McCarthy to be seen in the DVD bloopers.

Email: Sophie.Schillaci@THR.com; Twitter: @SophieSchillaci