As I already explained here, eyes are mirrors of our souls, they speak a langage that our mind and your heart understand instantly.
That's because our brains are divided in 2 parts : our "left brain" controls thoughts, and our "right brain" controls emotions. When you look at sometone, that person can instincly feel how you feel about her/him : your eyes are speaking for you. They speak about pain, joy, emotions, and thoughts. So if someone looks at you with his "right eye", his right brain, you feel his emotions, if someone is watching you with his left eye, you feel...uncomfortable, like he's analysing you. Cause that's exactly what that person is doing.
Eyes are the reason why however bad Loki can act, we adore him, because his eyes are talking pain to our hearts. And eyes are the reason why, however breathtaking Thranduil is, you immediatly feel he's dangerous, when he looks at you straight in the eyes, you feel uncomfortable. Thranduil is almost always watching people with HIS left eye, he's not feeling, he's thinking.
look what happens here, even if the studio's lights are lighting the right side of his face, his left eye seems...bigger, brighter than the right one !
Now look what happens when Thranduil is really feeling somethin, anger : he's suddenly watching with his right eye :
Of course, that's something directors know very well, you can see Peter Jackson filmed Thranduil's left face almost all of the time.
Look at his eyes the very first time you can see him :
You see how's he's watching Thorin with his thinking-left eye ? even if the right one is lightened, this right eye is not staring at Thorin, it squints a little, it's not the watching eye.