It just...shouldn't take over the story, really, and so on.

See: Tamora Pierce. She tends to describe characters as we meet them, more or less (immediately or within a few lines of dialogue, mostly). Through the descriptions, though, we tend to learn a bit about the new character--their social status, personality, current emotional state, stuff like that. It's also at least sometimes filtered partially through another character's observations, and used to tell us a bit more about said other character and what they notice (can be linked to backstory--for example, Trisana Chandler, who's from a Merchant family, will notice specifically if someone's wearing more or less expensive fabrics, etc, and do so in such a way that you see that she picked up a whole lot of knowledge, was probably even taught it).

Here, I'll find an example.

And here we go! A connected pair of descriptions, from Daja's Book. They're of two of the four main characters, both pretty young at this point (and by chance, at least on my part, originating in directly opposite social classes. Might have been intentional on Pierce's part to place them in the same scene for this early set of descriptions; at any rate, these come from pages 16-18 in my copy). Both characters appeared earlier, with minimal description (a sentence or two here and there, in exactly the style of providing relevant details as the story moves along that you mentioned):

Sandry (Lady Sandrilene fa Toren):

[Sandry] was a slim, fine-boned girl, with bright blue eyes and a stubborn chin. Sunstreaks gilded her brown hair, tidily braided and pinned up under a sheer gray veil. Her overgrown was dove gray linen, sleeveless and plain but for a long row of jet buttons down the front. Jet buttons also twinkled atop her small, black shoes. Her puff-sleeved undergown was white cotton, woven so fine as to be almost comfortable in the stuffy heat of the day. She would have loved to trade this elegant mourning for just one of her light cotton dresses, but that would have shocked the nobles who housed her great-uncle and his companions on this long ride through Duke Vedris' [said great-uncle's] realm. [...] Instead, as long as she rode with the duke, she wore the clothes proper to her station and envied her three friends their freedom to wear colors and fewer layers as she herself did at home.

and Briar (Briar Moss, Sandry's non-biological brother):

Briar leaned against the tree and ate his grapes. Unlike [Sandry], he was dressed for comfort: he wore cotton breeches and normally went barefoot, unless one of their teachers forced him into sandals or boots. At five feet, he was taller than Sandry by a hand's length. He had the glossy black hair--worn short and rough-cut--almond-shaped eyes, and gold-brown skin of an easterner, but a thin-bladed nose and eyes that changed from gray-green to lime green pointed to western blood in one of his parents. He wasn't sure which of them it might be: he had never known his father, and his mother had died when he was four.

(Briar's doing better than some months earlier--if I remember correctly, he used to cut the sleeves off his shirts, not just run around barefoot!)

Anyway: she does take time to describe characters, at moments when it won't grind the story to a halt--earlier, for example, she mentioned that Sandry was dressed richly (in contrast to the other three kids), but she waited for a quieter scene to give the full picture. She also uses the descriptions to tell us useful/interesting things: Sandry's station constraining her, the fact that it doesn't do so in the same way when she's home (and hints that her great-uncle the Duke doesn't mind it); Briar's much less stable childhood (the description only hints at it, but it's mentioned elsewhere and a large part of who he is and how he develops--he was on the streets from his mother's death onwards, surviving as a thief in a little gang), the fact that he's still adjusting to his new life but is actually beginning to relax into it (that's admittedly more noticeable if you've read the previous two books--I have, so I can see the contrast between the boy who complained about the new ways of dressing [claiming parts of it were uncomfortable or unnecessary, and doing things like, as I mentioned, cutting off his sleeves] and this version of him who still has to be forced into shoes but doesn't seem to have really modified his outfit, or to be shifting uncomfortably in it). These are useful descriptions, and she uses them and others like them to build a very complete, colorful world. Side characters will often get much shorter descriptions; it really depends on role, and the observing character, and if they're being used to provide contrast, like the paragraph I just found in a later chapter comparing the hosting noble to Duke Vedris! She uses it to, through Sandry's eyes, comment on...well, basically on them being very different types of people, and to strongly imply that Sandry--of a very similar social class, even!--approves more of her great-uncle's version. Even better, this is a recurring theme throughout the entire series.

Alright. I do, in fact, intend to do something else in the next couple hours, so I'd better wrap this up...but hopefully I've made my point with this impromptu mini-essay on Tamora Pierce's writing style. Detailed descriptions aren't always bad! As with many, many things, it's all in how you use them and where they're placed :)

~Z, who did, in fact, just take the opportunity to dive back into one of her favorite fictional worlds for longer than intended. Why? Because fun!

I agree with everything hS said, except that I'd advocate a best practice of giving your audience some idea of what your protagonists look like in their first appearance, while keeping it sensible for the POV and narrative voice you're using. If there's an observing character who would think "they remind me of a porcelain doll," that's fine. It's a simple description that gives a lot of information in just a few words, which can be very effective. But, like hS says, the words you choose and how you use them do imply things about the observer as much as or more than the observed, so be careful how you frame your description—especially since connotations may vary. Your observer may like dolls, but some people find them really creepy.

Also, if a more distant narrator is being extra-poetic about a particular character, that might very well raise Suvian flags for me.

But yeah, describing a character certainly shouldn't be immediate if there are more important things for the audience to be paying attention to, such as the all-important establishment of "where are we, what's happening, and why should I care?" Your picture can be built up gradually, as the pieces become relevant. It doesn't take much at all to give people an idea. It can be as simple as choosing your nouns well and/or slipping in an adjective now and then while describing an action the character is doing.

It's really very much more about the how and why than the what.

~Neshomeh's two cents.

But the thing is, what should the character turn into after exorcism, since the NEGs I mentioned above are never on-screen?

https://ppc.fandom.com/wiki/Sword

What will the dummies turn into, since variables are not defined by the narrative by definition?

• Therapy. Many agents have willingly reformed themselves with the aid of FicPsych, including certain members of FicPsych!
• Medication. Logicillin may prove useful. Anti-Lustin may also be used if the agent isn't a full-blown Suvian.
  -- There actually was an idea for glitter dialysis bopping around for a while years ago, but I don't think there's an actual published source for it (besides an offhand mention in one of my own missions). Obviously it would be impractical as a means of permanent de-Suification; perhaps, like real hemodialysis, it must be done periodically to maintain a suitable glitter level.
• Monitoring. The DIA may require particularly high-risk cases to wear a Sue-tracking anklet, either temporarily or permanently. This can be a tad humiliating and runs the risk of encouraging angst, but it has the benefit of providing the agent in question immediate feedback if they start to glitter, get OP, emit Aura of Smooth, etc.
• Community support. It's tough to be an ex-Suvian in Headquarters. At least one informal Sue Support Group has been known to exist. It's possible that FicPsych has since adopted the idea and started a formal one.

Any of these may be use separately or in combination, depending on the individual case (i.e., what makes the most entertaining story).

Also, I must note that claiming a character has been instantly cured of Suedom without doing any actual work to improve himself would be, ironically, pretty Sueish. Definitely take his powers away if that's the story you want to tell, but remember that Suedom is far more about the execution of a trait or ability than the thing itself. {= )

~Neshomeh