See How Much Johnny Galecki Transformed for His 'Roseanne' Return

Johnny Galecki showed Big Bang Theory fans the range of his acting skills on Tuesday when he reprised his role on Roseanne after more than 20 years.

The actor, now best known for his role as Leonard Hostadter, got an early break on the original run of Roseanne. He played David Healy, boyfriend of Darlene Conner and younger brother of Mark Healy, who dated Darlene's older sister, Becky.

Mark Healy shares some characteristics with Galecki's Big Bang Theory character. Both of shy and awkward, with a subdued, quiet intelligence. However, the two are different in many key ways as well.

While Leonard is a people-pleaser who plays by the rules, David is more whimsical. This is ultimately why his relationship with Darlene ended, as he went off to pursue volunteer work opportunities while Darlene was left to care for their two children.

What's really striking is the difference in appearance between the two characters. For years, audiences have grown used to seeing Galecki as the clean-cut, unassuming nerd who lives with Sheldon Cooper. He can appear sloppy at times, but he is always clean-shaven with big, thick-rimmed glasses. Leonard generally wears faded t-shirts, hoodies and canvas jackets, occasionally trading them in for more business-casual looks whenever necessary.

David Healy, on the other hand, has the air of a distracted introvert. In Tuesday's new episode, he retained some of his '90s charm in a denim jacket and a patterned sweater. He also appeared in another scene wearing a thick flannel shirt, matching the blue-collar aesthetic of Roseanne, but bearing no resemblance to the hipster world of Big Bang Theory.

As David, Galecki also wears no glasses. He has five o'clock shadow coming dangerously close to turning into a full-blown beard -- more evidence that he doesn't put much stock in his appearance. Galecki is like a completely different person on Roseanne, without a trace of Leonard to be seen.

While Galecki got back into his Roseanne persona this week, it wasn't too long ago that Sara Gilbert, who plays Darlene, was gearing up to appear on Big Bang Theory with him. Gilbert played Dr. Leslie Winkle, Leonard's on-again off-again girlfriend in the first two seasons.

Johnny Galecki is back on @RoseanneOnABC tonight#Roseanne #ABC #Family #Comedy pic.twitter.com/sNRnUF9OWh

— Michael Fishman (@ReelMFishman) April 17, 2018

In real life, Galecki and Gilbert dated for a time as well. They were together until the late '90s, when Gilbert told him she was gay. She has been in a relationship with Allison Adler since 2002, and came out publicly in 2010. However, the two stars remain friends, exemplified in their continued on-screen chemistry.