See Kaley Cuoco, Johnny Galecki, Jim Parsons, and the rest of the Big Bang Theory cast before they found their inner geeks on CBS
Johnny Galecki: Then
Before finding his inner geek on The Big Bang Theory, Galecki was best known for playing David Healy on Roseanne. But he also had roles in some other 1980s and '90s classics. In 1989, at age 14, he starred as Rusty Griswold in National Lampoon's Christmas Vacation, alongside Chevy Chase, Beverly D'Angelo, and Juliette Lewis.