 According to A. Scott Berg (qv)s memoir "Kate Remembered",  Katharine Hepburn (qv) was already chosen for Mary but they had  trouble casting Elizabeth. At one point Hepburn, who had by then  been nicknamed "Katharine of Arrogance", suggested that she play  both roles. Supporting player John Carradine (qv) asked, "But if  you played both queens, how would you know which one to upstage?"  She was not amused at the time but roared with laughter when  retelling the story years later.