This Is Forty's storyline revolves around Knocked Up couple Pete (Rudd) and Debbie (Leslie Mann)'s attempts to overcome the irritations of middle-age.
Rudd explained that he could not pass up the chance to work with Knocked Up director Judd Apatow and co-star Mann once again.
"It's been surreal… The dynamic that Leslie and I have is pretty great," he told The AP. "There's already a familiarity with working with each other."
He continued: "It's been fun to play that part again. I really liked doing Knocked Up, so to play that part again but to do more stuff is great."
Megan Fox, Melissa McCarthy, Jason Segel and Chris O'Dowd round out This Is Forty's cast.
Fox recently praised fellow This Is Forty stars Segel and O'Dowd as both "handsome" and "very funny".
This Is Forty is expected to be released in 2012.
> Megan Fox is hilarious in Knocked Up spinoff, says Leslie Mann
Watch Paul Rudd in the Knocked Up trailer below: