The actress signed on to star in Sir Ridley Scott's blockbuster, alongside Russell Crowe and Cate Blanchett, just a week after Richardson died from a brain injury following a skiing accident in March.
The acting legend was set to tackle the role of Queen Eleanor of Aquitaine, mother of the folklore hero's nemesis Prince John, played by Oscar Isaacs.
But now it would appear Redgrave is not yet ready to return to work - Dame Eileen Atkins has confirmed she has accepted the part in the 2010 movie.
And Atkins admits she is "terrified" about the project.
She says, "They keep rewriting the script and changing my lines quite dramatically, so I'm terrified of forgetting them."