This is something that has me a bit confused, although I am probably overthinking it. When provided with circumstances that will test a trait, which side of the opposed trait do you test? For example, in KAP 5.1 on page. 83, the example used is that Sir Bors is in the woods and sees a beautiful maiden. The GM has Sir Bors test his Lust trait. But why not the Chaste trait? You can argue both ways. The Lust trait is tested to see if Lust overcomes him, or the Chaste trait to see if he resists temptation. The next example in the book has Sir Ambrut initially test his Merciful trait to see if he spares the cranky uncle. Why not initially test Cruel to see if Ambrut is overcome by cruelty? How do you choose which side to initially test?

You wing it.

The only guideline I can think of is that you should think of how the scene is framed and let that tell you which. In general if its about not doing something then use the Virtue, if its about doing then use the Vice. I'm not sure if that makes sense to anyone else but that's how I do it.

If the trait (or the opposite one) is 16+, roll that first. Otherwise, I generally let the players choose, which one they wish to roll first. This gives them a bit more agency in the story progression, too.

However, sometimes I straight up ask them to roll a certain trait. For example, in the last session, they saw this unearthly beautiful elf woman. Lustful rolls all around, an instinctual response to see if they are attracted to her. With a more mundane woman, I would have allowed them to choose between the traits.

I usually make these opposed tests. So if they see a beautiful maiden like in the example of sir Bors I would call for a test of Chaste vs Lust. They will act depending on the winning result.

In some cases I will exchange a trait for a passion (or other trait) depending on the situation. If Sir Bors has a lady love you could have him roll his Lust vs Amor or Love(Lady) in stead of Chaste. As the case with sir Ambrut a Hate passion may exchange the cruel trait.

I go for which makes a better story. This may call for a few questions. In the first case (transferring the idea to a PK), does the knight have a love? Has be been actively wooing her? If wife, does he actively make time in the campaign to signal his love of her? And so on. In that case, I would rule chaste first to see if his love comes to mind before he goes further. Otherwise, Lustful would be first.

Now, YPMV, and so I allow a Passion to influence traits as well as skills, so the knight could try to inspire his chaste because of his love.

If he has no love, then I agree with Morien in that notable traits have priority.
Cornelius also has a good idea which I have used in the past, before I switched to my method above. But, it is your campaign.

Thanks guys; I really appreciate it. Hopefully, my campaign will start in a week or two. I'm getting excited.

If he has no love, then I agree with Morien in that notable traits have priority.

Unless the Love is 16+, too, I would still call for Lustful 16+ first. However, that being said, I would likely do it as a Opposed Resolution as Cornelius said. Same if there is a reason that Loyalty (Liege) would come into play, etc. However, I very seldom do directly opposed pairs, although now that I think about it, there is absolutely no reason why not. It seems like an elegant solution, by-passing the whole 2-step process. In a nutshell:

1. Roll opposed resolution of both Traits.
2. If one Trait wins (or the opposite trait fumbles), go with that. Check is/may be awarded. Notable Trait PKs cannot go against the successful roll, but others can; the penalty is a check in the opposite trait.
3. If neither succeeds (nor fumbles), feel free to choose. No check is awarded.

Those are distilled from the Trait roll results (Table 4-1) of the main rulebook (5.1) with some of my own interpretations. I am not sure about the punishment clause in step 2, though. It seems to me a bit strange that you get a check when you succeed in the opposite Trait. I can understand why it is there, to poke the PKs into playing their high Traits (which they presumably want to play), but it would also open the door for a bit of minmaxing: Lets say I have a Just 15 and Arbitrary 5. I succeed in Arbitrary, but I choose to act Justly. Hey, presto, a check in Just. But if I had succeeded in Just, I would have gotten a check in Just, too. Why bother to even roll at this point? Just give the check based on the action chosen, and occasionally FORCE the rolls & results. If you are targeted by a Faerie sleep spell, you have to roll Energetic vs. Lazy, and Lazy wins the Energetic/Lazy contest, then you WILL fall asleep and get a check in Lazy. (If both fail, I would just say that the spell works since Energetic failed, so the character sleeps but doesn't get a Lazy check.)

Good luck. Let us know how it goes.

The rules allow that on a successful first trait roll you can still choose to act in accordance with the opposite trait. Therefore, I usually say that the player should roll first on the trait that describes how they want the character to act. That way, you can usually (assuming neither trait is 16+) choose to give in to temptation regardless of roll, rather than being allowed to choose to resist in spite of succeeding on the tempted trait.