https://www.looper.com/1792437/johnny-galecki-the-big-bang-theory-sheldon-cooper/

It's hard to imagine Johnny Galecki playing anyone but Leonard Hofstadter in "The Big Bang Theory," but the co-creators initially wanted him for another role.

Johnny Galecki could not have pulled off Sheldon. Ironically, his Leonard role ended up being more supporting which he hadn't wanted despite being billed first.

He was a bigger star at that time. He added a lot to the show, even being a supporting part. More than Raj, for sure.