Lord Of The Rings: 10 Smartest Changes Peter Jackson Made From The Books
4. Making The Eye Of Sauron A Physical Entity
In Tolkien's writing its never clear what form Sauron took during the War of the Ring. After he's destroyed in the Battle of the Last Alliance, he loses his physical form, but whether he regained some kind of body is unknown. Gollum - who was tortured by Sauron - tells Frodo that he only has four fingers on his black hand, which implies Sauron does have a physical body.
But more often than not Tolkien referred to the "Eye of Sauron" rather than Sauron directly. This has kept people guessing whether or not the "Eye" was merely a metaphor for Sauron's will, insight, and influence or whether it represented his physical manifestation.
In the book we never actually see Sauron; he never speaks directly and the only evidence of his existence is the impacts of the actions done in his name. It was a pretty brave choice for Tolkien to never clearly define his main antagonist, but somehow it made the idea of Sauron all the more menacing.
For obvious reason Jackson felt the need to show at least some kind of manifestation of his main villain. The decision was made to turn the Eye of Sauron into a literal and physical thing, atop the tower of Barad-dûr. Interestingly enough, Jackson also planned for Aragorn and Sauron to fight at the end of the Return of the King.
All the footage of Aragorn fighting the troll outside the Black Gate was filmed with Sauron, but Jackson later changed his mind and digitally replaced the Dark Lord.