Tolkien:By the end of the Third Age, Sauron's physical power is so diminished that his presence is confined to the tower of Barad-Dur. However, he is still immensely powerful in his ability to watch and predict the movements of his enemies, and his understanding extends so far that he could metaphorically be referred to as a Great Eye, constantly watching and planning his next move. And because his gaze is so piercing, it is sometimes likened to fire.
Peter Jackson:Okay but...hear me out...what if Sauron was literally....a Great Eye.
Peter Jackson:What if he was literally a giant eyeball on the top of Barad-Dur.
Tolkien:That's not quite what I inten-
Peter Jackson:Made of FIRE. A LITERAL GIANT FIERY EYEBALL.