Check out the making of documentary for THX 1138 - there’s bunch of episodes in which George goes full nerd on Duvall who looks progressively more confused. It is especially funny if you compare how Coppola worked with Duvall on set of Rain People (George did the documentary about that).

He still has that magnificent head of hair. (how I envy that)

Both guys look so incredibly young, too. I’m not sure, but I think his pal Francis Coppola helped him land an actor of Duvall’s quality, despite being a completely rookie director IIRC.