Johnny Galecki played Leonard Hofstadter for 12 seasons in The Big Bang Theory, but there was another actor from the sitcom who almost portrayed the role: John Ross Bowie. As one of the two original characters from the sitcom (the other one being Jim Parson's Sheldon), Leonard was always going to be an integral part of the Chuck Lorre and Bill Prady sitcom. Had things turned out differently during the casting process, it would've been Bowie and not Galecki landing the gig.
Fans of The Big Bang Theory are more familiar with Bowie as Barry Kripke — Leonard and Sheldon's colleague in Caltech who debuted in season 2. Originally a plasma physicist, Kripke eventually became a string theorist like Sheldon. Over the years, he's attempted to work with the boys, but more often than not, things go awry between him and Sheldon leading to frequent petty battles. Kripke is best known for his case of rhotacism — resulting in him pronouncing "r" and "l" as "w" making for some funny dialogue every time he's on the sitcom.
Galecki was always going to be a part of The Big Bang Theory as Lorre personally reached out to him to offer a part. Known for his sitcom credits in Roseanne and one episode of Blossom (with Mayim Bialik), the actor has made a name for himself in the genre, so it's not surprising that creatives of the CBS project wanted him for their show. They originally wanted him to play Sheldon, but Galecki turned down the gig because he felt like the character was too similar to his previous roles. Instead he went for the role of Leonard. The thing is, CBS was already holding auditions for that part, with Bowie coming in to read for the role.
In his blog entry from 2012, Bowie revealed that before auditioning for Kripke, he tried for different parts in The Big Bang Theory — including reading twice for Leonard. The actor didn't reveal much about what the process was like for him trying to score a role in the sitcom, but he added that if things turned out differently, he and Kevin Sussman (who played comic book shop owner Stuart) would've been The Big Bang Theory's main characters, Leonard and Sheldon. While Bowie didn't score the leading role for the long-running sitcom, he was a fixture in the popular series, starring in a total of 25 episodes with the last one during the penultimate outing titled "The Change Constant" where he pranked Sheldon for supposedly winning the Nobel Prize in Physics for his and Amy's work in Super Asymmetry.
No one knows how The Big Bang Theory would've turned out if Bowie was cast as Leonard. However, while the actor has remarkable comedic timing, which made for some of the funniest moments throughout the show's history, Galecki played the part so effectively that it's difficult to imagine anyone portraying the role. It's also worth noting that Galecki's chemistry with Parsons was integral in the show's success considering that their characters' friendship was the foundation of the series. So aside from simply playing Leonard that way the character was written, Galecki's rapport with his co-star was also a deciding factor when casting him as Leonard.