Eh, is this really such a big deal? I haven't read the book in years, but doesn't the emperor have only a few lines in total?
I'm having a really hard time remembering Walken in anything other than Privateer 2 and that one Adam Sandler movie but I don't think the role calls for anything really extravagant or even has the dimension to; José Ferrer was far more memorable in the opening they made up for him than in anything he did for the rest of the film where his job was basically to look pretty as his dynasty dies.
Walken is kind of an odd choice but it should be fine as long as Villeneuve chose him and it wasn't forced on him.
One thing that comes to mind is that all of the actors who played the role are too old. The book states that Shaddam was 70 but looked 40; Ferrer was 71 during filming, Giannini 58, and Walken is 79. The role really is more about the image and dignity, rather than the personality (which is what, a stubborn old man?)