He's even said as much:

I throw the spaghetti against the wall, see if it sticks, see if it's al dente. I know the material, and then, of course I want to try it a bunch of different ways. Give the director so much to work with, so many levels of emotion, so many levels of cartoon. I'll do them all night if they let me.

To me that is him finding it so fun he just wants to try each part every which way he can and find what works, because he's truly enjoying trying it out because he wants to try it a bunch of different ways, not because he's being told to.

Not "I have a specific thought about how this works", or even "I know what the director needs from me", but "This is so fun I want to be wild with it and see what I can do!". Which to me is what makes Carrey awesome as Robotnik.