Coppola was brilliant with his casting.
Castellano was coming off of an Oscar Nominated role and was possibly the best known actor at the time (After Brando, of course)
Brandos career had peaked, then ebbed and nobody wanted to work with him.
De Niro, Pacino, Caan, Duvall, Keaton were all either unknown or little known.
Sterling Hayden still struggling from his blackballing in the 50's.
Even Talia Shire was quite a ploy - Thinking he was going to be fired, he signed her so someone in the family could make some money!
Brandos career had peaked, then ebbed and nobody wanted to work with him.
...
I think I read that the studio wanted Ernest Borgnine for the role. Can you imagine?
Godfather and Godfather II are among my top ten movies of all time. They are morality plays, imho, and never grow old.
Think of the Bible and Shakespeare. Love, greed, hatred, revenge. Every human weakness. Love for family. I usually watch both once a year.