The only caveat I would put forth is that among many Shakespeare scholars, there is an assumption that he did not write Hecate's big scene. Someone Else did, but we don't know whom. It is one reason that Hecate is often cut from The Scottish Play.
If there are real and pseudo Shakespeare scholars among the auditioners, this could be an issue.
Think of the cool steely stiletto of Angela Lansbury in The Manchurian Candidate. Or the opposite--Cruella DeVille. Or a combination of both. My point being that you can go in any number of directions with a character like this so long as she remains magnetic and in control.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher