If you've ever wondered why Leslie Mann shows up in so many Judd Apatow movies, you're not really paying attention. Leslie Mann is Judd Apatow's wife. But before you scream nepotism on this particular occasion, consider that Apatow originally wanted Jodie Foster for Debbie, sister of the female lead (played by Katherine Heigl). When the Jodie Foster situation didn't work out, sure, Judd probably turned to Leslie like, "Honey, can you give me a hand with this? I'm making a movie and..."
Leslie Mann for Jodie Foster in Knocked Up (2007)