By the time fans first met Johnny Galecki as Leonard, he had been working professionally for years. As a kid, he told people he wanted to be either an actor or a witch when he grew up (via Behind the Brand). His parents didn’t see it (hence the acting career) and tried to encourage other interests. “But I was very, very stubborn,” Galecki recalls. Then he saw an 8-year-old Ricky Schroder on The Champ and realized this was a career he could start right away. And he did, went to the theater, found an agent, and moved to Los Angeles.
He appeared in television films and feature films throughout the 1980s, including 1989’s National Lampoon’s Christmas Vacation. In the 1990s, he had roles in television series, including 1990-1991’s American Dreamer, 1991’s Blossom, and 1992’s Billy. But when The Big Bang Theory premiered in 2007, everyone wondered why he was looked so familiar, probably remembered him best from “Roseanne”. He played Darlene’s (Sara Gilbert) boyfriend David on this long-running sitcom from 1992-1997.
However, for all his success, Galecki had already given up acting by the time The Big Bang Theory was in development. “My career hasn’t been smooth sailing,” he told Entertainment Tonight. “Right before ‘Big Bang,’ I was in Wisconsin studying plumbing.”
https://www.looper.com/1005296/johnny-galecki-was-on-his-way-into-a-very-different-career-before-landing-the-big-bang-theory/ Johnny Galecki was on his way to a very different career before he landed The Big Bang Theory