Contagonist: Why?

I have an intellectual understanding for the need of a Contagonist, but I don't have a practical understanding. Can anyone give me an example of a Contagonist in a well-known film? I've tried to figure out what a Contagonist's motivation might be. Why do they try to thwart or deflect the Protagonist? I can understand why an Antagonist would act like that, but I can't understand what motivates a Contagonist to do so. Are they deluded? Do they know something the Antagonist doesn't? Do they have mixed feelings? Everything I've read seems strong on theory, but not particularly enlightening in practice. Is the Contagonist supposed to be a subtle character who's not as easily quantified as a Protagonist or an Antagonist? Is the concept blurry because the character itself is supposed to be blurry?

First off, there is no need for Contagonists to WANT to tempt or deflect the protagonist or others, it is just what they do. In other words, intent is not needed for a Contagonist to function (the same is pretty much true for all objective characters in the overall story).

The simplest example of a Contagonist I can think of is the little devil that sits on someone's shoulder giving them all sorts of bad advice. Of course, he is balanced out by the Guardian angel. It's important to note that a Contagonist represents temptation and hinder, but is not necessarily applied in any specific direction. The Story Goal gives a useful barometer for an author to use, but that not required for the character to do his job. In Star Wars, Darth Vader is the Contagonist. He deflects the efforts of the protagonist Luke by killing Obi-wan, but also deflects the antagonists' efforts by squabbling with Empire officers, and even suggesting that Gran Mof Tarkin let Luke et al leave the Death Star with the tracking device, which allows the rebels to find and exploit the Death Star's weakness before the Empire blows the Rebel's base to bits. Darth also represents temptation, as in 'here's what happens to you if you give into the temptation of the dark side of the Force.' In subsequent films, he more actively tempts Luke to join him.

The Contagonist need not be blurry, but may appear to be less obvious if it's functions are shown using subtle storytelling techniques.

What is the difference between Contagonist and Antagonist?

The differences between the Contagonist and the Antagonist are best understood by looking at their component character elements.

At the motivation level, the Contagonist represents Temptation and Hinder, whereas the Antagonist represents Reconsider and Avoid/Prevent.

Where a contagonist might lead one astray and impede one's progress, an antagonist would make the argument that one should completely reconsider one's plans while actively acting to avoid or prevent them from taking place.

The same type of relationships between the Contagonist and Antagonist exist in the other three levels of character elements -- Methodology, Purposes, and standards of Evaluation.