Protagonists need someone to stand in their way, someone who will challenge them and create the conflict necessary to drive a story forward. The Antagonist provides one aspect of this opposition.
It has been written elsewhere that one of the more unique aspects of Christopher Nolan's stellar film Inception is that there is no Antagonist to be found. Dominque Cobb (Leonardo DiCaprio) and his merry band of mind thieves operate without being hampered by a discernible evil baddie. Some of have even gone so far as to suggest that its lack of such an important character is a sign of why it is a "bad movie".
Sidestepping the insanity of link-bait, the first step towards understanding who the Antagonist comes with precisely defining the objective function of such a character.
The Antagonist prevents the efforts to reach the Story Goal.
Since every story is really an intensive look at the problem-solving process that goes on within the human mind, it only makes sense that there should be some End-Game--some resolution that is being sought after. This resolution is known as the Story Goal and is actively pursued by the Protagonist of a story.
The Antagonist wants those efforts to fail.
Define the Goal first. Protagonist and Antagonist will follow.
In Inception the Story Goal is to get Fischer Jr. (Cillian Murphy) to realize he needs to Break Up His Father's Empire. Everyone in the story has some connection to this Goal, but there is only one person who actively drives the efforts towards it, regardless of consequences, and that's Dom Cobb. Thus, Cobb is the Protagonist of the piece and when Fischer sits on the shore with who he thinks is Peter Browning (really it's Eames) and tells his friend the epiphany he has reached, those efforts have ended in Success. The problem has been resolved.
So now we know the Goal, and we know the Protagonist. Who then prevents Cobb from reaching this Goal?
The first, and most obvious choice are the heavily-armed agents that Fischer has developed to defend his own subconscious from intrusion. Cobb wants in. They want him out. They want failure. Sure, they're not as sexy or charming as a typical Villain, but then, does every story require such a character?
The Antagonist is not always a Villain.
Many make a big deal about how important a Villain is to a work of fiction. Where would Die Hard's John McClane be without Hans Gruber? Or The Dark Knight's Batman without the Joker? Or Potter without Voldemort? Some would argue that the success of these stories can be attributed in some respect to how prominent and charismatic these Villains are.
But the real reason why many feel these baddies are more effective than the subconscious defenders of Inception is because these Villains encapsulate what is known as the Archetpyal Antagonist. Representing not only the drive to prevent the Goal from being reached, but also the drive to force the Protagonist to reconsider such efforts, the Archetypal Antagonist is a more familiar obstacle to general audiences.
But the focus in developing a sound structure should be on resolving problems, not identifying and utilizing recognizable patterns of storytelling.
Complex Characters are not so cut and dried.
Complex characters break away from the norm in that they attempt to give the audience something new, something unexpected. The problem with Archetypes is that they often incite a feeling of sameness, an "I've seen this before" sort of feeling that comes when an audience knows exactly where a character is coming from. Sometimes this predictability is OK, maybe even warranted, but more often than not, it isn't. That's where Complex Characters come into play.
Designing the Unexpected
One way of avoiding the tedium of the same comes from dividing up the characteristics of a typical Archetypal Antagonist among different characters. Instead of piling them all up in one comic-booky representation of a bad guy, as is the case with Hans Gruber or the Joker, spread them out in patterns that are unfamiliar or unique to a particular story.
In Inception the drive to prevent the Goal from being reached is given to the well-weaponized defenders. That much has already been clearly established. However, the drive to force the Protagonist to reconsider the efficacy of their Goal--the other half of the Archetypal Antagonist--goes to Ariadne (Ellen Page). On more than one ocassion, she works diligently to get Cobb to see the error of his ways and perhaps rethink his efforts to resolve the problem.
Does Ariadne want the mission to fail? Up until she's actually in Fischer's mind, she does. Even then, she still continues to badger Cobb, forcing a rethinking that stalls the Protagonist and delays the eventual resolution. Now, she has other characteristics that offset this opposition and prevent her from being seen as a truly "bad guy", but seen in the context of achieving the Goal of a story, she provides that mental obstacle.
An Antagonist also forces the Protagonist to reconsider.
Most bad guys, particularly Villains, take on this second characteristic of an Antagonist. Think of Grand Mof Tarkin in Star Wars and his threats of blowing up Alderaan, or the King of England in Braveheart and the public execution designed with the sole purpose of getting William Wallace to reconsider his stance on Scotland's freedom.
Inception avoids this convention. The problem, of course, is that because the sentries of Fischer's minds are only challenging Cobb physically and really don't exhibit any other functionality in the story, they can come off as mindless drones. Structurally this is not a problem, but such one-dimensional characters can be a source of consternation and confusion for many. For the rest of us, they fulfilled their dramatic roles with precision and expertise.
Fulfilling functions, instead of time-tested patterns
The quest for the Antagonist in Inception is simple: identify the Goal and determine who actively prevents it and also who actively forces reconsiderations of such an effort. Both Ariadne and the sentries of Fischer's subconscious adequately serve this dramatic function, thus providing the resistance needed to accurately depict the human mind and the process it undergoes in solving a problem.
Advanced Story Theory for this Article
In Dramatica, the Archetypal Antagonist is driven by the Motivations of Prevent (or
Reconsider. The Protagonist maintains the opposite characteristics of
Consider. Archetypal Characters are usually used in simply structured stories that would rather focus on flashy special-effects and action than character development.
Inception is rare in that it tries to do both.
By splitting up the Motivations of Prevent and Reconsider into different Players, the requirement for a complete story is still met while simultaneously delivering something an audience has not seen before, or at the very least, is not overly familiar with.
It should also be made perfectly clear that when speaking about the Protagonist and Antagonist of a story, only the Objective--or Overall Story Throughline--is considered. Cobb's drive to get back home is certainly another aspect of the story, but one that lies in a different place, namely the Main Character Throughline. Dramatica sees both throughlines occurring in a complete story. One is no more or less important than the other. When trying to determine the Antagonist of a story, it becomes important to focus the attention only on the dispassionate Overall Story as both Antagonist and Protagonist work in this area.