By whatever name they’re called—villain, antagonist, bad guy, nasty character—the characters who stand in opposition to your protagonists are integral to your novels.
He, she, or it—human antagonists are more satisfying than machines or non-humans when we’re talking actual villains—the oppositional character of your story plays a major role.
The baddie sets up a great deal of your lead character’s challenges. He stands against the protagonist, actively works against him, determined that your main character not succeed in his endeavors. The antagonist not only works to undermine the lead and keep him from achieving his goals, but the antagonist actively works to achieve his own goals, which may parallel the lead’s or be the opposite in every measure.
You must create a relationship in opposition for your story—both protagonist and antagonist cannot succeed. If your main character, your protagonist, achieves his goals, the antagonist will have failed to stop him. If your protagonist fails, the antagonist will have been successful.
They cannot both win, though theoretically both could lose. If neither achieves his goal but at the same time prevents the other from achieving his, then you have a tragedy or a post-modern tale.
If both do somehow win, it’s because they’ve joined forces or were not in true opposition after all. However, while writing everything but the ending, you’ll have to maintain that the characters are in opposition. Otherwise they are not protagonist/antagonist and there’s another antagonist hiding in your story.
Sometimes the bad guys win. And win big. And sometimes they don’t get what they want but see to it that the protagonist doesn’t get what he wants either.
Of course, sometimes your villain, to the sound of cheers in your mind, is soundly trounced, while in other stories the hero limps home, alive and successful, only without a leg or his girl or the ideals that set him on his path to begin with.
A character can also stand in opposition to something in himself or to something in nature, something other than a person. My concern in this article, however, is a human or human-like character acting as antagonist.
Freedom is when one hears the bell at seven o’clock in the morning and knows it is the milkman and not the Gestapo.Georges Bidault (via smokeandsong)