John Mark Galecki started his path to fame in the miniseries 'Murder Ordained'. On that same show was JoBeth Williams and future Roseanne star John Goodman. John also was on Roseanne, and prior to his role in The Big Bang Theory
, he was best known through his role as David Healy.
After Roseanne, John Galecki would do a streak of acting in films that included movies 'I Know What You Did Last Summer', 'The Opposite of Sex', 'Bounce', 'Suicide Kings', 'Bean', and 'Vanilla Sky'. However, in 'Vanilla Sky', John Galecki had a minor role.
Eventually after doing several more sitcoms, and side theatre roles, John Galecki would land his most famous role to date. Leonard Hofstadter. This a character in the television show, 'The Big Bang Theory