I think casting directors and others in the business understand how to look beyond someone’s physical appearance—if that can be changed to suit the character. You can look at all the supporting actors in Outlander to see this in action, too—Gavin McTavish looks NOTHING like Dougal’s description in the book, nor does Tobias remotely resemble how DG described Frank/Black Jack. But they nail the essence of the characters perfectly and you instantly believe them. I suspect the actor they chose for Ian, who is so vastly different from the book description of him, will be the same. If a hair color, or an eye color, or particulars about a character’s build are essential, that stuff can always be changed. Cait his straight hair, Claire has curly hair. That’s why I don’t think they’ll cast a redhead as Brianna—they’ll cast the actress who embodies Brianna and send her to the salon.