In most genres, this character or entity has a goal that is the opposite of the protagonist’s goal. If the protagonist wants to uncover a mystery, the antagonist must be dead set on keeping the mystery unsolved. If the protagonist wants to win the girl, the antagonist must want her for himself or be so opposed to the protagonist’s pursuit that he will do anything to stop the couple from getting together.
The antagonist should also have something deep within that drives him toward his goal. The emotion or underlying belief system must be as strong as the protagonist’s for the stakes to be high. He endorses the flip side of the thematic argument.
The antagonist can be a group or organization but there must be someone who leads the group for the reader to focus on. A virus does not make a compelling antagonist unless someone is walking around intentionally trying to infect people with it. It makes a good overall story problem, but the person trying to stop the virus needs someone equally in favor of letting the virus take its course.
A good resource is Jessica Morrell’s Bullies, Bastards and Bitches: How to Write the Bad Guys of Fiction.
The more heated the struggle between the protagonist and antagonist, the more intense the conflict is for the reader.
The important thing in any genre is that the antagonist not be a cardboard stereotype placed there because the script called for it.
Depending on the genre, there can be a “friendly” antagonist who has good intentions and acts as the catalyst that prompts the protagonist to make a necessary change. This type of antagonist is often employed in a Comedy, Road Trip, or Literary tale.
Antagonists don’t have to be evil lords or serial killers. They can be concerned friends, parents, coworkers, or people who think they are acting in the protagonist’s best interest but who are misguided in their beliefs.
But what if your story doesn't have a bad guy?
Some story structures have antagonistic forces without having an actual “bad guy.” The antagonistic forces must be strong enough to make the reader doubt whether the protagonist will gain what he wants in the end.
The British are masters of subtle conflict and subtle antagonism, both comedic and dramatic.
Antagonistic forces can be internal demons, a system, a disease, a system, or nature.
There can be several people working at cross purposes in the story in the subplots. The richer the complications, the better the story, unless you add so many subplots that the core plot becomes hard to follow.
Check out Story Building Blocks: The Four Layers of Conflict for more information on antagonists by genre and Story Building Blocks II: Crafting Believable Conflict for building antagonists by personality type.
More advice on crafting antagonists:
Levels of Antagonism
Bad Guys in Romance
Villains Are People Too
Taking Your Antagonist On A Date
Antagonist Conflict Scenes
Diana Hurwitz is the author of Story Building Blocks: The Four Layers of Conflict, Story Building Blocks II: Crafting Believable Conflict, Story Building Blocks III: The Revision Layers, and the YA adventure series Mythikas Island. Her weekly blog, Game On: Crafting Believable Conflict explores how characters behave and misbehave. Visit DianaHurwitz.com for more information and free writing tools. You can follow her on Facebook and Twitter.