Develop the Antagonist

The bad guy, the villain, the adversary
More properly defined is that they are literary the opposite of the protagonist, they're the character who oppose the desire of the protagonist.
How to keep the antagonist engaging and interesting?
The basic is the antagonist is the force that the most actively trying to impede or to challenge the growth and the goal of the protagonist.
Be careful that sometimes you're setting the goal (the goal might be to stop war) so the protagonist will want to stop the war and the antagonist want to prevent the war for being stopped. So you start with those double negatives but always think that you want the audience to see the antagonist be the one who prevent it, against it, stop it from happening.
Psychopaths, purely self-motivated, internal motivation to re-consider the protagonist.
The antagonist can be in mentally world, not only in physical world.
They can both have the same goals but in different ways.
They don't need to be balance and it can make the end shows the battle has become much more interesting.
A good villain should be almost the dark mirror of the protagonist. They should be a very very similar people. Really.