It turns out Stiller not only didn't originally want to play the role of George's father, he'd never even heard of the show. In fact, they actually cast somebody else. When he finally did accept, the dad was supposed to be meek and mild. Fortunately, his own sense of what the character should be took over and the Frank Costanza role is now a part of TV history. 

I hope he's finally found "serenity now."

Jerry Stiller Initially Passed on 'Seinfeld'