If you notice he is fumbling with tying the fly as his hands seem shaky. Some of the associate directors wanted a more experienced fly fisherman to play that part and they held tryouts in Montana. In the end Redford chose the old guy with the shaky hands. All the tryouts were fly fishermen but he chose the one who had the most difficulty in tying the fly. He thought it was more realistic to the story. I think he got it right.