I remember Bob Odenkirk and Michael McKean having a conversation on a Better Call Saul Blu-ray where he said how nearly every year there's people surprised when yet another comedian shows that they can act. Odenkirk and McKean are as good examples of that as anyone.
Vince Vaughn is legitimately good in the criminally underrated second season of True Detective and he's funny in recent seasons of Curb Your Enthusiasm. Apparently, when you get that dude on HBO, he's good.
I'm sure that Coppola expected Brando/Kurtz to be in reasonably decent shape but in the context of the movie, he's gone completely nuts so I can believe that Kurtz has let himself go. However, I agree that they do seem to be trying to hide him in the shadows so I'm sure they wanted Kurtz to have...