Jerry O’Connell (Vern) had never really acted before – he had just one commercial under his belt. When he auditioned, he recognized Rob Reiner and said, “Aren’t you the guy on channel 5?” At the time, the local channel 5 had been airing reruns of All in the Family at the time and Jerry recognized Rob as Meathead.
(BTW, John Lennon’s cover of this song is pretty good)