Apocryphal (but probably directionally correct) story I've heard is that Redford didn't understand why Mike Nichols didn't want him for the lead. He really needed an explanation. So Nichols tries to explain the mood he is going for with the character and says something like "You know that feeling when you are out at a bar and you ask a girl to go home with you and she turns you down?" And Redford is confused and is like "um...no...wait, is that a thing that actually happens?"