The builders of fictional worlds, in science fiction and fantasy, come in two chief types, the meticulous and the speculative. The meticulous world-builder delights us by thoroughness of invention, the speculative by surprisingness. For the former, and for readers of the former, much of the pleasure of a fictional world arises from the working out of details; for the latter, and for the latter’s readers, what especially delights is the quirkiness or oddity of the invention, and the “what-if-the-world-were-like-this” questions so aroused.The master of meticulous world-building in fantasy is of course Tolkien; in science fiction it’s Kim Stanley Robinson. Speculative world-building is more common, because it doesn't require so much detail: there is no genuinely meticulous world-building at less than 750 pages or so, I’d think. But that doesn't mean that the speculative type is easier to do well: it requires an instinct for the telling detail, the most distinctive and provocative ways in which a given world differs from our own, and an equally shrewd instinct for what doesn’t change. Keith Roberts’ Pavane, Ursula K. LeGuin’s The Left Hand of Darkness, Hope Mirrlees’ Lud-in-the-Mist, all strike me as especially wonderful examples of speculative world-building.There are bad ways to read both kinds of world-building. You cannot reasonably expect a meticulous work to be lively all the time; you cannot reasonably expect a speculative work to be perfectly consistent in all its details. But some readers have unreasonable expectations. The fair-minded reader of the meticulous text will deal graciously with longueurs; the fair-minded reader of the speculative text will smile forgivingly at inconsistencies. The masters of the meticulous are also skilled at limiting drearinesss; the masters of the speculative avoid capriciousness. (Aside: no one has ever handled the need to fill in the details of an imagined world more brilliantly and charmingly than Susanna Clarke, in the footnotes of Jonathan Strange and Mr Norrell.)Both kinds of storytelling are adaptable to different media, but the requirements of meticulousness incline such makers to books rather than movies — though the rise of the long-form television series is pretty well-suited to meticulousness. The speculative is perhaps more dependent on style than the meticulous, but that style need not be linguistic: it can also be visual.All these thoughts are prompted by Fantastic Beasts and Where to Find Them, which I saw last night and absolutely loved. I could say a good deal about various elements of the film, but in this post I just want to focus on the world-building, which was, I think, superb. I really do believe that if I hadn’t known that J. K. Rowling wrote the screenplay I would have guessed, because it bears the marks of her particular gift, which is the speculative.Because there are seven Harry Potter books, and now a series of ancillary media — the little books of Fantastic Beasts and Quidditch Through the Ages, the Pottermore website, a play , this new movie and the four (!) sequels to come — it’s natural to think of Rowling as one of the meticulous world-builders, spooling out stories from a secure repository of well-worked-out details. In fact the Potterverse isn't very meticulous at all, and has ten thousand holes in its fabric, as readers have pointed out from the very beginning.No, it’s the curious, provocative, stimulating detail that Rowling specializes in, and here that gift is manifested best in Newt Scamander’s suitcase, which wonderfully extends an idea Potter readers learned about first, I think, with the Ford Anglia that Mr. Weasley enchanted in Harry Potter and the Chamber of Secrets: in the magical world, objects can be bigger on the inside than they are on the outside. In Newt’s suitcase the creatures that others fear but that he loves and wants to protect (from “the most vicious creatures on the planet: humans”) find safety and affection. And, in Eddie Redmayne’s wonderful portrayal of Newt, we see an awkward, mumbling, socially uncomfortable man transformed, when he enters the little Ark he has made, into an expansive and confortable figure, a skinny ginger Noah. Watching all this just made me happy.I’m not convinced that the four movies to come will work as well as this first one did, and I’m especially nervous about the Grindelwald story that will clearly be a prime plot driver. But Newt Scamander proves, to my surprise, to be an utterly captivating protagonist, and I will eagerly await his further adventures — without expecting the strict consistency and precision of world-making that the meticulous craftsmen offer.