It's writing, it's production design, the casting, it call comes together in such a harmonious way. Each scene is rich with subtext and detail, and it does such a fantastic job of showing rather than telling.
In trying to write the character of a maniacal psychopath it's hard not to take a few pages out of the good doctor Lecters book. I see so many movie villains now a days with traces of his marvelously malevolent personality. I love the way we're introduced to him as a character.
First off we're given snippets of his backstory, how he killed and ate his patients for seemingly no reason. Secondly we're informed by an orderly escorting Clarice to Lectors cell of the unbridled brutality that Lector inflicts on others, even while he's still incarcerated. We're told that we aren't allowed to get too close to his cell, and that we can't tell him anything other than the information that we've been cleared to disclose. Every shred of information that Lector learns about you is ammunition he can use against you. With that in mind it makes it all the more chilling when before engaging Clarice in conversation, Lector smells the air for a moment and tells her what skin lotion she uses, and the perfume she wore the night before. It's chilling, invasive, and remarkably thrilling. While I don't think I have the writing chops to crank out a character with the complexity of lector, it's still extremely interesting to watch the ways he unnerves us, and take that as inspiration.