Johnny Galecki only acted in NBC shows.
Johnny Galecki . He is known for playing David Healy in the ABC sitcom Roseanne from 1992 -- 1997 and Dr. Leonard Hofstadter in the CBS sitcom The Big Bang Theory since 2007 . Galecki also appeared in the films National Lampoon 's Christmas Vacation ( 1989 ) , Prancer ( 1989 ) , Suicide Kings ( 1997 ) , I Know What You Did Last Summer ( 1997 ) , Bookies ( 2003 ) , and In Time ( 2011 ) .