Apparently every film or spin-off that has anything to do with Knocked Up will feature an actor/writer/director in a paternal role. The great Harold Ramis played Seth Rogen's stoner dad in Judd Apatow's 2007 smash and now the filmmaker is about to lock Albert Brooks into his upcoming feature, a spin-off of his sophomore release, to play Paul Rudd's father.
It's an interesting development because Brooks doesn't sign on to just anything. He hasn't appeared in a film since 2005's Looking For Comedy In The Muslim World, which he also wrote and directed, and hasn't taken part in one since 2007's The Simpson's Movie, in which he lent his voice to the Russ Cargill character. My guess is that the two are friends and revere each others work and that the strength of Apatow's past features (or perhaps this new script) turned Brooks onto the idea of working on the film.
Deadline notes that Brooks is still in talks to join the cast, which just yesterday appears to have added Megan Fox. I can totally see him playing Rudd's father as they are close enough in physical appearance (and comedic stylings) to pull it off. Whether or not the deal goes through, Brooks can next be seen in Nicolas Winding Refn's Drive, starring Ryan Gosling.