One of the key problems to solve when writing stories that contain magic is how it works. I think it’s a bad idea when magic is too driven by the needs of the plot—you see this a lot in superhero stories where the heroes’ and villains’ powers seem to change depending on where in the story they are. (I’m looking at you, Infinity War, where at one point the super-powered Vision and Scarlet Witch need to be rescued by Captain America, Black Widow, and Falcon, three characters whose abilities are meant to be within the range of extraordinary human capability.) It makes victory and failure both feel a little false, harder to enjoy, and while a story can pull us through if we care enough about the characters, it has the effect of making them a little too thin, easier to see through.
At the other extreme are stories that spell out in extreme detail how the magic works. Computer games and tabletop games like Dungeons & Dragons do that sort of thing by necessity, because they’re trying to balance the mechanics of magic with the mechanics of fighting, and because the magic has to be systematic and predictable so it works the same in a variety of contexts. But really I think magic, real magic, shouldn’t be quite so predictable; if it is, then it’s just Clarke’s “sufficiently advanced form of technology.”
What I’ve tried to do with Uprooted and now with Spinning Silver is to create magic that feels justified, even in a relatively concrete setting, without being mechanical. Fairy tales have a kind of magic that doesn’t follow strict technical rules, and yet it satisfies because it generally requires the characters to work for it, which is the fundamental truth, I think: magic has to come at a price. If it’s free, if you don’t have to feel the cost the characters pay for it, then it’s easy for the magic to feel like the hand of the writer, giving their characters an easy way out. And magic should generally make the characters’ lives harder rather than easier, I think, at least up to the resolution of the story. That’s certainly the case for Miryem and Wanda and Irina, and Agnieszka from Uprooted, all of whom end up in more rather than less trouble and danger because of their magic.