Details are little quirky parts of your character that are only helpful in rare or obscure circumstances. Details are ways to fill out the peculiarities things that make your character unique.
During Character Creation, you can add a few Details if you’d like, but you should try to keep the list short (at least to start). You don't have to take them right way, either - details don’t make a huge mechanical difference, and so can be added as they come up.
Details represent very minor Characteristics. They're meant to add a few interesting and potentially useful twists to a character.
Details should follow these guidelines:
- Details are Small: A detail is a unique and specific part of a character, but it's also a small thing, like an affectation or a hobby
- Details aren't obviously useful: They are things that you might notice about a person, or they might talk about, but that don't come in handy very often
- Details aren't Tangles: Details shouldn't be things that get you into trouble. “Bit of a problem drinker” might seem minor enough, but it's much more likely to cause problems than be helpful, and probably deserves to be a Tangle instead of a detail, for instance.
Let's talk about The Princess Bride.
Remember the scene when the Dread Pirate Roberts gets into a duel of intellect with Viccini? How he had spent years developing an immunity to Iocane Powder, apparently just in case a situation like this came up?
That's a Detail. Immunity or resistance to all poison is far too broad; that's probably a Special Ability. Immunity or resistance to all snake venom, or to neurotoxins, or to all synthetic poisons is likewise too broad. Being immune to one specific poison works really well, though. My character is immune to Death Dust, or Black Lotus Powder, or Iocane Powder all work well.
There's a Constructed Language out there called Lojban; it's a language intended to be universal based on logical principles, and has a wonderfully complete grammar. It hasn't gained a lot of ground in the world, but it does have a few dozen speakers in the world. Knowing Lojban might make a quirky Detail for a character; it says something about them, could come in handy in a somewhat bizarre circumstance, and provides no obvious general benefit on a day-to-day basis. If, however, during the game you run into a fellow Lojbanist, it could be very useful, establishing an immediate connection and common thread.
Knowing Lojban is a neat Detail. Language is largely hand-waved in unnamed, but knowing a strange, obscure, or subculturally significant language is a Detail. In the Star Trek setting, knowing Klingon is just a part of your character's background. On modern day earth, it's a Detail.
We all know someone who always carries a pocket knife or a multitool. As indicated in the Stuff article, if it makes sense for a character to always have a piece of equipment, then they probably have it. But if there's something small that you always have, that might be a detail. This is best reflected with details like Always wears sunglasses or Never leaves home without their Leatherman.
This is different from listing an item in your stuff because normal circumstances where you might assume they'd forgotten it or left it at home don't apply. For instance, if someone is forced to run out of their house in an emergency, they probably won't be stopping to grab a bunch of stuff, but if the character has a Detail like Inseparable from their Kindle they will, in fact, have that funny little tablet with the ad-riddled OS on their person.
You can gain new details at any time during play. Like a Connection, the GM can give you one whenever they think it's appropriate. Likewise, if something happens that you think might make a good addition to your character, you can ask the game group if you can add it to your character (“Anybody mind if I throw Just loves Anime on my sheet, since I've referenced it six times in two sessions?”). You don't generally need to spend Advancements to get new Details.
From a mechanical standpoint, on the rare occasion that a Detail can be of some help to a check, it grants you Synergy. Details can't be burned or pulled for checks, but if more than one Detail applies, then you get Synergy for each one.
Details can come up in other ways. A detail might allow you to notice something you wouldn’t otherwise, or get drawn into a situation for any of a number of other reasons. It might also allow a check you wouldn’t get otherwise.