Ryan Seacrest is known as a lot of things - slick “American Idol” ringmaster, hotshot radio deejay, ubiquitous E! channel host - but funny guy’s not one of them. Which makes his scene-stealing cameo in “Knocked Up” such an amusing surprise. Even more so because he’s the butt of the joke.

Seacrest sends up his pompous image in the Judd Apatow-directed comedy about an ambitious entertainment reporter (Katherine Heigl) whose life is flipped when she learns she is pregnant. She takes ...