Johnny Galecki acted in an ABC sitcom for five years.
John Mark `` Johnny '' Galecki -LRB- born April 30 , 1975 -RRB- is an American actor .. 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 .. David Healy. List of Roseanne characters#David Maurice Healy. Roseanne. Roseanne. Dr. Leonard Hofstadter. Leonard Hofstadter. The Big Bang Theory. The Big Bang Theory. Galecki also appeared in the films National Lampoon 's Christmas Vacation -LRB- 1989 -RRB- , Prancer -LRB- 1989 -RRB- , Suicide Kings -LRB- 1997 -RRB- , I Know What You Did Last Summer -LRB- 1997 -RRB- , Bookies -LRB- 2003 -RRB- , and In Time -LRB- 2011 -RRB- .. Prancer. Prancer ( film ). Suicide Kings. Suicide Kings. I Know What You Did Last Summer. I Know What You Did Last Summer. Bookies. Bookies ( film ). In Time. In Time