Carl Reiner wrote the first 13 episodes of the a show that he could star in, about a comedy writer like he’d been for Sid Caesar. They made the pilot and it didn’t work.
His agent wanted him to try and do it again, since he had all these scripts and he complained to a director friend that he didn’t want to make another failure.
The director, Sheldon Leonard (who Chuck Lorre named the Big Bang Theory characters after), said, ‘it won’t fail, well just find a better actor to play you.’ A couple weeks later, Leonard sent him to see this Broadway actor by the name of Dick Van Dyke.
Mary Tyler Moore didn’t want to audition at all, because her last few auditions went poorly and she was thinking about quitting.
I can’t recall where Reiner knew Rose Marie from, but he wanted her for Sally and after she read the script, she told him you should bet my buddy to play Buddy. And they did.