In both the three Tolkein books, and in Jackson's films, Sauron remains alien & opaque, a legend plus a flaming eye. ROP's Sauron appears in flesh and blood, can alter his appearance, can interact with people. ROP uses this to give us Sauron as the ultimate abusive partner

Sauron, in ROP, is shown as "the great deceiver". His way of working is to sneak into your life, to appear to be what he is not (a hunan warrior, a learned and immortal elf) and then slowly encourage his target to make decisions that corrupt the self. It is masterfully done for three reasons: