Okay, excuse me, but I still don't get it. I mean, if Elphaba is played by a white actress, then she might have the mannerisms of a white girl and therefore she will be "white" and not "green." Since green girls do not exist in real life, we cannot affix a "white" stereotype to them either! If Saycon had mannerisms that we would typically associate as "black" sass, then who cares? Elphaba can have black stereotypes, she can have white streotypes, she can have Asian stereotypes... it doesn't matter! But when saying that having black mannerisms and not "green" mannerisms what you are inferring is that Elphaba should have "white" mannerisms which is not only wrong but it has a hint of racism in it!
"You know, a little orphan girl once told me that the sun would come out tomorrow. Her adopted father was a powerful billionaire, so I supressed the urge to laugh in her face. But now, by gum, I think she might have been on to something!"
--Reefer Madness