Good writers will flesh out lush, elaborate worlds with round MCs and supporting characters, each with their own back stories and motivations. But then, out of the shadows, comes this thing. This 2-dimensional face of evil (with horns!), thwarting your beloved protagonist at every step, at all costs, for no other reason than that this dude (or gal) is a real piece of work.
Unless he's a demigod living in the fiery depths of Mordor, this antagonist will need humanizing and fast. I know! *Scrabbling in the screenwriter's toolbox...* Why not have him pet the dog? You know, that adorably sweet, loveable, fuzzy-eared next door neighbor dog with the gimp leg? And for no other reason than that Antagypoo likes dogs. He really does.
And if not the dog, maybe save a baby, or decline squishing the grasshopper at his picnic, or stand up for the stuttering boy in class, or . . . you get the point.
This is not a quick fix. But it will help with antagonists and rough-around-the-edges protagonists alike, especially near the beginning of a book, when we haven't learned enough about the characters to really care about them.
Can you think of famous book or movie characters who "pet the dog?" How about when Harry Potter saves the boa from captivity in The Sorcerer's Stone? A pseudo "pet the dog" moment.