I seem to remember Marcia suggesting the idea, George being nervous about telling Alec. Alec being initially a bit miffed and then was relieved that he wouldn’t have to stretch his acting abilities with the dialogue which he hated and then sat back and watched all the money roll in while moaning about the film whenever someone was brave enough to mention it. He did suggest Seb Shaw for Anakin though which was nice because he exuded a similar spirit in those few scenes his eyebrows were allowed to attend.