In many ways antagonists are more interesting characters than protagonists, especially in relation to the plot. Basically they are the ones stirring the pot; all the order either in the world or in the life of the protagonist is disrupted by the antagonists. Famous examples of antagonists include Sauron from The Lord of the Rings, Lord Voldemort from Harry Potter and Darth Vader from Star Wars: The Original Trilogy.
The different types of antagonists are not as straight forward as the different types of protagonists. In my post on protagonists I was very clear about the fact that you need to be limited in the number of protagonists that you have in your story. You may end up with some prominent minor characters that support the protagonist, but when it comes to antagonists you have fewer limits placed upon you.
Essentially at any point in the story, any character can become a pot stirrer. Best friends, minor characters, crowds even. Antagonists are not limited to characters, like the ones I mentioned above, who are hell bent on destruction, domination and evil. This admittedly is a viewpoint most commonly found in genres like fantasy. In crime for example, it would be the criminal, or if you have a twisted viewpoint it might be the authorities who are the antagonists. In romance it might be the cheating lover, or the irritating in-laws.
For example, both Ron and Hermione in the Harry Potter series are antagonists at one point or another. Hermione starts out as one until she becomes Harry’s and Ron’s friend. Ron in The Goblet of Fire becomes jealous and unsupportive of Harry right when Harry needs his best friend the most. He comes around eventually, but in between the emotional toll on Harry makes the situation he finds himself in even harder to handle. In both cases, in classic fantasy plotting, it takes either a troll or a dragon to turn the characters into or back into protagonist supporters. Outside of fantasy you will have to find some less spectacular plotting devices.
This variety in antagonist types means that when it comes to casting your story with characters, what you might find is that the most interesting characters are actually the ones who cause the most trouble. Their motivations might be based on a greater variety of possibilities that your protagonists. They aren’t just surviving; they might be plotting and planning, and being more deliberate in what they do in your story.
However the trick is with antagonists is not to make the reader want them to be the protagonist. They may appreciate the deviousness of the antagonist, but in truth what you want to create is an antagonist that is either redeemable so that the audience can allow themselves to be sympathetic with them, or you have to create your characters with an flaw that no amount of evil brilliance would allow the audience to accept them and want the protagonist to claim victory.