Thank you so much for your reply. Of course, I would say "He was famous...." I just wonder... Isn't the world "whom" as a object for "knew"?? In your clear explanation "He was famous..." the words "we know" are gone. So, I just feel confused with..."who we knew was famous for his plays" in this sentence, what's the object of knew?? Sorry to bother you again...
I can't make "who we knew was famous for his plays" sound right to me.
Even so: We knew he was famous for that. We knew it was he who was famous for that. It was he who we knew was famous for that. This guy, who we knew was famous. In this he was famous, so he is the subject. What you know is that he was famous for doing something.