"Very early on during the design phase of Crusader Kings II, I realized that the way we usually model relations between states in our games would not suffice." This is something of an understatement: in Europa Universalis, there are a few hundred nations at most, and most of them never interact with one another. In CKII, there could be hundreds of people who need to interact within a single kingdom, thousands in the game world at any given time, and thousands more people who have died or are yet to be born.
Fåhraeus describes a clear plan for making this work: "What we needed for this sequel was a simple, unified mechanic; easy to understand, yet deep and meaningful. Thus, we came up with the Opinions of Crusader Kings II; one single value summed up from a number of clear reasons why someone would like or dislike you (e.g. holding desired titles, refusing a request, or just plain old personality chemistry.) Opinions are unilateral, meaning the feeling is not necessarily mutual. The AI, of course, relies heavily on the opinion values, meaning it can act intelligently and in character. The end result turned out to be one of my personal favorite features in Crusader Kings II."
These Opinions work on a scale of -100 to +100, and are most commonly used for understanding when vassals might rebel. A powerful duke may hate his emperor, and you'll see why -- maybe the emperor has just acceded to the throne (an immediate -20) or that emperor has seized titles from other vassals and is viewed as a tyrant.
"All these numbers are visible and easy to access via tooltips. As a player, your goal would be to placate that duke, perhaps by granting him more land or sending money, or perhaps to isolate him from his allies by befriending them instead.
With the idea of multiple unilateral opinions set, Fåhraeus started adding them to the game. "The original Crusader Kings... (is) one of progenitor systems of the new Opinion mechanic, though it was far more limited. For Crusader Kings II, we took many of the traits from the original game, but with some modifications. For example, I wanted to have the Seven Deadly Sins and the Seven Virtues as traits. There are other changes too, of course. For example, some traits are now congenital and can be inherited by your offspring."
What I found fascinating about this is that Fåhraeus simply "wanted" to have those particular traits, so he had them added to the game, and they worked. In fact, they work extremely well at multiple levels: they add a medieval, religious flair to the game, but they also effectively give the characters more personality and set them at odds with one another -- a Lustful duke will automatically be wary of his Chaste queen, even if they get along at every other level.
This is what made me particularly interested in the testing process. My theory was that this was an intense process, with each trait examined and tweaked for optimal performance. I was wrong.
According to Fåhraeus, "...Actually the opinion system never really got much criticism and did not undergo any major revisions from the first implementation in Crusader Kings II, although the testers helped the development and came up with many new opinion modifiers and helped balance their magnitude. In my experience, quite often you know a feature works really well when people who test the game do not talk about it much. Beta testers usually do not bother with praising well-functioning mechanics, being far too busy to find faults in the game."