You must know everything about your characters, even to the change in their pockets.
Knowing their soul is paramount.
Show, don't tell. Instead of talking about your character, show who they are.
And remember, go for the extremes. If possible, make them David and Goliath extremes.
1. Heavy Antagonists
Go for the extremes, make your antagonists the deadliest antagonists.
(Eg. The most dangerous dictator, the deadliest cancer, etc.)
Even better, if possible and suitable for your story, make the antagonist both heavy and hero. We love to love them and love to hate them simultaneously. If you have such a character, you can have a spectacular screenplay.
(Eg. Cersie Lannister)
2. Flawed Heroes
Every major character should have a flaw.
Audiences don't want to see Mr. Dudley Do-Right.
One of the best flaws is Obsession.
Your hero should want his goal so badly that he will battle any equally obsessed antagonist to achieve it against all odds.
Nothing is as important as achieving that goal.
Obsession is the hallmark of every great movie and every great hero.
The hero must have something at stake. They must not be in a position where they can shrug their shoulders and walk away from the problem. They are going to see it through to the sweet or bitter end.
3. One Heavy and One Hero
2 equally formidable antagonists cancel each other out.
Pick 1 to be the most powerful, formidable antagonist. He or she can have henchmen or subordinates to do his dirty work.
Same for the protagonist. There should only be one main protagonist role.
If there is another main character in the same camp, he or she should be subordinate in the life of the story.
This character is more reactive than active.
In all "buddy" films starring two characters, one is always dominant.
4. Motivation
Every classic antagonist has one of two motivations: Greed or Power. Period.
You must identify his or her wants, and transform those wants into drama.
No matter how evil, or seemingly evil, the antagonist is not without his good side. He certainly has a good reason for behaving the way he does.
Most murderers, except for maniacs, have their own logic, and in a way almost sympathetic reasons for taking a life.
They have reasons they think are good, that rationalize their want for power or greed.
(EG. The pedophile in Fallen Angel truly believed he was loving unloved children. That was his justification for wanting power over them. Captain Nemo wanted to rule the world by harvesting the sea to save the planet from itself.)
Every hero should have one consuming motivation, which is to overcome the obstacle, be it emotional or the forces of the outside world.
5. Exciting Heroes and Heavies
When "right" and "entertaining" come together, you're on the right track towards building exciting heroes and heavies.