Too often writers focus on developing their protagonist and forget that the antagonist who puts that character into conflict must also be well developed.
The first step in creating a fully-realized antagonist is to be aware that he or she does not necessarily have to be an opponent. He must be a character who has a want or need that clashes with the protagonist’s want or need. The two characters should not both be able to realize their desires. Thus the conflict.
Too often, in my opinion, writing instruction books give the impression that all an antagonist needs is some good quality to flesh him out and make him not one dimensional. The typical example is the serial killer who loves his pet dog or dogs in general or little children. He goes out of his way to save a puppy or feeds his dog a special treat before heading off for the next murder.
I don’t see that as sufficient character development. Instead, an antagonist must be true to his own want or need.
For example, I believe the serial killer can be thoroughly reprehensible—evil, through and through. But his struggles need to be real. He needs to kill or to foil the detective out to catch him. He needs to have his own character arc—struggles that push him forward and motivate him to make choices, along with consequences that refine him or weaken him.
Of course, the antagonist doesn’t need to be evil. He simply must be the principle foil for the main character.
If the antagonist is the business partner of the protagonist, he might believe that the only way to make ends meet is to accept a client our hero finds unacceptable. They both want to make the business succeed, but the main character wants to be free of any questionable associations. He wants a growing business, but he needs to be a man of integrity. The antagonist, on the other hand, needs to be successful, no matter what the cost. These two are far from enemies, but their differing needs pull them in opposing directions.
When beginning a novel, knowing the main character is imperative, but knowing the antagonist is equally so. It is the antagonist who provides the counterbalance to the main character even as he throws up roadblocks and deepens the conflict. Good stories inevitably have good antagonists.