Terfel sang the entire opera with his hair pulled down and hiding what was supposed to be an empty eye socket. Only in the curtain calls did he smile and pull back his hair to reveal a living eye underneath. It occurs to me that it must be extremely difficult to perform anything with only half your vision. You need both eyes to focus properly. Maybe that's why Terfel stood motionless so often. He had a few quick movements, but nothing tricky. I imagine Eboli in Don Carlos has the same problem -- unless her eyepatch has a pinhole opening.