The English word antagonist comes from the Greek - antagonist?s, "opponent, competitor, villain, enemy, rival," which is derived from anti- ("against") and agonizesthai ("to contend for a prize").
In the classic style of stories where the action consists of a hero fighting a villain/antagonist, the two may be regarded as protagonist and antagonist, respectively. However, the villain of the story is not always the same as the antagonist, as some narratives cast the villain in the protagonist role, with the opposing hero as the antagonist. An antagonist is usually neutral character which may represent a threat, being mean, or obstacle to the main character by its existence and not necessarily targeting him or her in a deliberate manner.
Examples in both film and theatre include Sauron, the main antagonist in The Lord of the Rings, who constantly battles the series' protagonists, and Tybalt, an antagonist in Romeo and Juliet, who slays Mercutio and whose later death results in the exiling of one of the play's protagonists, Romeo. In stories, a convention of antagonists is that their moral choices are less savory than those of protagonists. This is often used by an author to create conflict within a story. However, this is merely a convention, and the reversal of this can be seen in the character Macduff from Macbeth, who is arguably morally correct in his desire to fight the tyrant Macbeth, the protagonist .
Characters may be antagonists without being evil - they may simply be injudicious and unlikeable for the audience. In some stories, such as The Catcher in the Rye, almost every character other than the protagonist may be an antagonist.
An aspect or trait of the protagonist may be considered an antagonist, such as morality or indecisiveness.
An antagonist may not always be a person or people. In some cases, an antagonist may be a force, such as a tidal wave that destroys a city; a storm that causes havoc; or even a certain area's conditions that are the root cause of a problem. An antagonist also may or may not create obstacles for the protagonist.
Societal norms or other rules also may be antagonists.
A character once a protagonist can turn into an antagonist under extremely negative circumstances. Examples include Anakin Skywalker from the Star Wars franchise who turns from Jedi to Sith. A supporting protagonist can become an antagonist by betraying a main protagonist.
An antagonist is used as a plot device, to set up conflicts, obstacles, or challenges for the protagonist. Though not every story requires an antagonist, it often is used in plays to increase the level of drama. In tragedies, antagonists are often the cause of the protagonist's main problem, or lead a group of characters against the protagonist; in comedies, they are usually responsible for involving the protagonist in comedic situations.