I read somewhere the phrase, ‘Inside every antagonist, there’s a protagonist waiting to come out’ ( obviously a variant of the saying ‘inside every fat person, there’s a slim person trying to come out’).
The antagonist, of course, makes the story nearly as much as the protagonist. If you have a weak or insufficiently motivated antagonist, it massively detracts from the tension, and tension, as so many advisors on writing craft constantly hammer home, forms the main interest of a plot.
Of course, an antagonist does not have to be evil or wrongly motivated. An antagonist can just be someone on the other side about a particular issue, and have strong arguments for doing the things s/he does to frustrate the will of the protagonist. Then, the situation can be as true to life as it often isn’t, when the rights and wrongs of a situation are wholly clear cut, with the baddies writ large – though reading about that sort of situation has its own appeal.
I have often thought how in Shakespeare – with the exception of his early, unfair depiction of Joan of Arc – part of his outstanding greatness is his capacity to depict everyone’s point of view fairly.
For instance, this is true of one of his greatest antagonists, Edmund in ‘King Lear’. He is shown to be motivated in his appalling villainies and his determination to usurp his legitimate brother’s place, by Gloucester’s insensitive treatment of him, leading to his obsessive jealousy of Edgar. Gloucester even jokes coarsely about his mother in front of him to the Duke of Kent. Then Gloucester adds that he has been abroad and will be sent away again…
At the end of the play, on hearing how Goneril and Regan have died through their rivalry over him, he says, ‘Yet Edmund was beloved’. That is a tragic phrase; it shows what underlay his dismal scheming and brutality. He trusted no-one; but he had a great neglected need to be loved.
Tybalt in ‘Romeo and Juliet’ is often described as the antagonist, and his motivation appears to be more simple than as so often in Shakespeare. He seems to be merely violent, and to hate the rival family unthinkingly.
An antagonist can be honest but misguided, or charming but flawed, or an estranged former ally. What s/he does have to be – even if s/he isn’t a person, but an impersonal force like the weather or a societal rule – is strongly motivated and a fit adversary for the protagonist.
It can be difficult to enjoy a book when you wholly side with the Antagonist, as I did with Mary Renault’s books about Theseus. There, the Antagonist is female power as personified in the Great Mother, and I was very sorry that Theseus as the personification of patriarchy inevitably triumphed as regards power. He did not, of course, triumph as regards his personal life, ending up an empty shell after the sacrificial death of his Apostate Amazon and his murder of his wife Phaedra (incidentally, that was an astoundingly inefficient choke that he used; done efficiently it should have rendered her unconscious in ten seconds appproximately, not in minutes; but that’s the Sportsfighter in me speaking) .
Then there is the story where the Antagonist can triumph – the Anti Hero, and the one where the Protagonist is his or her own Antagonist. Perhaps s/he does things under an unconscious influence or is even haunted by his own Doppleganger; as In ‘Dr. Jeklyl and Mr. Hide’ and the main character in Chuck Palanhuik’s ‘Fight Club’.
Sometimes, an antagonist truly makes a series, for some readers at least, becoming a favourite character.
I remember when I read ‘The Mortal Instruments’ to my daughter, I never thought the later books were the same after the larger than life antagonist Valentine was killed off. I found his habit of jumping through a portal with a contemptuous jibe a brilliant feature (besides, I was sad that he was never depicted as repenting before going to his final account).
On entertaining Antagonists, I recommend Rannie in Robert Wingfield’s ‘The Legend of Dan’ series. I do relish an Antagonist with a sense of humour. Also, there is Harpalycus in Rebecca Lochlann’s ‘Child of the Erinyes’ series. He is so foul it is funny, and he revels in his wickedness. What he most keenly relishes, after many centuries of body hopping, is causing as much misery as he can on the three main characters, Aridela and her two rival lovers. 
I have said before, that sometimes the Antagonist is unfortunately more interesting or even more sympathetic than the Protagonist , and even if s/he fails to win the lead characters’ love, or take over the world – or whatever it is s/he wants to do –the said Antagonist anyway takes over the book.
This is what I felt about Clement Willoughby (the original of Jane Austen’s own beguiling Willoughby, perhaps –as we know from Mary’s quotes from Evelina that Jane Austen had read it). He is far more entertaining than the sententious Lord Orville, and protests eternal devotion to suspicious heroine, the problem being that he never quite gets round to proposing and makes the occasional half-hearted effort to abduct her.
On Jane Austen, I never thought that Wickham was quite up to the part. That may just be me, and perhaps a villain of that sort doesn’t need much strong motivation, and just drifts from one self-indulgent escapade and heartless seduction to the next. While general hatred towards Mr. Darcy is part of his motivation, it isn’t somehow made convincing, and his seduction of Lydia is just part of his general self indulgence rather than something he is strongly motivated, though it does further the plot. I suppose this weakness, this tendency to undertake mean schemes and then to be bought off from them, is meant to be all part of his villainy, but somehow he is a disappointing villain.
Another interesting point, to return to what I said at the beginning of this post, is the fact that sometimes exactly who is a Protagonist and who an Antagonist can be unclear. It can even depend on perspective, as according to the point of view, their roles can be interchangeable. It is to a particular example of this that I return in my next post.