Okay! So today we're going to talk about villains, or antagonists for the politically correct, and since there's already been a ton said on them, I am going to keep this short and sweet.
I read on someone's blog somewhere (I wish I knew where, or I would totally link it) that you should think of your antagonist as the protagonist who failed.
Let's take Star Wars as an example. Luke Skywalker is a Jedi Knight. He is learning how to use the Force for good and stop the evil empire. Darth Vader is also a Jedi Knight. He is also wildly talented, but instead, uses the Force for evil. They are mirror opposites, and when we find out *SPOILER ALERT FOR THOSE OF YOU WAKING UP FROM COMAS* that Darth Vader is Luke's father, it makes their connection all the stronger. Luke's father failed to resist the temptation of the Dark Side. Will Luke be able to do the same thing?
This idea always reminds of a level in video games they sometimes make you play. You're stuck in a mirror world, and you have to fight your mirror self, often called "Shadow Link" or "Dark Mario."
That scene in The Empire Strikes Back? Where Luke is fighting Darth Vader in the cave, and then he realizes he's really fighting himself? That's how a really effective antagonist works. He doesn't have to be an evil cyborg Jedi Knight bent on destruction. Your antagonist can be a good person. He just has to show the protagonist how he's going to turn out if he fails. He sets up the consequences of failure.
In this manner, your antagonist has the most in common with your protagonist, even more so than the protagonist's friends. A cop chases after a serial killer. A cop upholds the law, the serial killer breaks the law. A doctor is fighting to save a patient from an unknown disease. A soldier is fighting enemy soldiers in a war. Each protagonist and antagonist are locked in a give and take struggle. It's what makes fiction so engaging.
Not all antagonists are a mirror of your character's darker self. Godzilla comes to mind. Some of them are just the pure evil, gonna crush you all sorts. But if if you're going for something deeper, consider how alike the protagonist and antagonist are. Consider having the protagonist seriously tempted with "the dark side".
And then show him the consequences of failure with the antagonist. This has a surprising effect of not only developing your protagonist better, but your antagonist as well.