The eye flicker is not deliberate; this is just a proof of concept/unpolished test.
I think it’s too fast so I’ll have to spread the transformation over multiple shots - perhaps have him go halfway to Sidious in this shot, and then take one of the later shots of the Sidious prosthetics and see if I can’t edit it to start out with him still only partially transformed.
OH MY GOODNESS! Looks better than the makeup!