Good Lord, it's everywhere. Most notably when Kevin's wife goes out with the business partners' wives and finally sees that they are actually demons in disguise. And the baby playing with the ovaries. If that's not enough, there's always every single facial expression Al Pacino has in this movie.
The slightly dumpy woman that guides Kevin through his time at the law firm seems as human as anyone. She's not overly attractive, like Tamara Tunie or Connie Nielson, and she isn't strictly implied to be super-smart with the law, though it could be since she works there. Unlike the others in their mask-drops, which have all shown demonic mouths and eyes, she just smiles a creepy, broken smile full of demonic teeth, as if to say that she doesn't need to drop the mask entirely. And then, when Kevin goes to meet Milton after Mary Anne has slit her own throat, she tells him in the exact same motherly tone she's used throughout the movie that he's waiting for Kevin and he'll take that fear away.
The scene where Mary Anne snaps and commits suicide by slicing her throat with a piece of broken glass.
Her gradual breakdown leading up to her suicide as a whole can be considered this.
The vagrants. Brrrrr.
The shot of Kevin stepping out into the bright - and empty - streets of New York City, where all of a sudden a city full of millions is just... this barren landscape of skyscrapers and trash-filled sidewalks. Nothing Is Scarier indeed. A visual representation of the path into Hell that Kevin must walk alone when he goes off to his final confrontation with Milton.
That shape-shifting sculpture in Milton's lair, with the alabaster figures who start copulating when Christabella seduces Kevin. Then Kevin puts a spanner in Milton's plans and the whole thing bursts into flames and the sculpted people start howling and thrashing in agony.
It's heavily theorised that the sculpture isn't a sculpture, but actually a Hellgate, or at least a window into Hell.
