See my “Abridged Guide to Intelligent Characters” for my attempt at a slightly extensive and very practical guide to writing the different types of really, genuinely, non-fake intelligent characters. The fancy technobabble almost never has anything to do with it from start to end; and while there are various techniques and tricks you can use to amplify the amount of intelligence you put into your writing to create a slightly more intelligent character than yourself, most of what goes wrong with conventional attempts to illustrate the “genius” stereotype is that it tries to cheat the entire process without any cognitive labor at all. Characters who are animated by an inner spark of optimization that tries to make their own life go well the way you’d try to make your own life go well in their shoes; characters who solve solvable mysteries and deploy cunning plots; characters who think so well that the reader actually learns something — none of these characters can be depicted without a lot of authorial sweat, while the character who knows twenty-seven languages and learns to play the piano in just five seconds requires no sweat a all. Doyles’s Holmes’s intelligence is, I would say, mostly fake; and I do think it possible to do much better.