The following I posted in response to a question on a LinkedIn forum I belong to in reference to the means of “chosing an antagonist.”
I like the idea that an antagonist personifies the hubris or fatal flaw of the hero. He shows the audience what the hero would become if he succumbed to his flaw and even embraced it.
Since Luke and Vader have already been mentioned, let's run with it. Darth Vader (and later the Emperor) has embraced the Dark Side and has become very powerful for it. Luke is being tempted by the Dark Side and is even succumbing to it to a degree. But, it is at that moment where he demonstrates that he would rather die by falling down the gas mining shaft than join Vader that he triumphs over himself and becomes the final hero whose actions lead him to his final confrontations.
What Bill said is correct, you need to see that the antagonist might have a shot at winning. He might even have a very good reason for winning.
Remember, though, that Darth Vader offered to finally end the entire war with Luke by his side, and they probably could have too. But, which was better, to give in and save hundreds to thousands of lives to establish a tyranny, or to continue to resist, lose hundreds to thousands of lives but let those lost along the way die in pursuit of the freedom of those who came after?
Provide both your antagonist and hero chances to grow and change becoming more of their defining character value. Remember that Han Solo is a lot more interesting (at least to me and many others to whom I have spoken) than Luke. Han starts at "Scoundrel" and becomes "General" and a hero of the Rebellion. But, ultimately it is when Luke faces the darkness within himself that he is able to become the true hero by triumphing over it.
