We can use footnotes in fiction, a lot of early novelist from the UK and Ireland used them, but they can be tricky to employ in modern fiction. Nowadays the footnote is largely associated with academia either referencing book titles or inserting extra information about a subject. The problem with footnotes is that it can become too easy to insert information and we can end up with as much in footnotes as the story itself. The second problem is that not all readers will read the footnotes so they need to add to the story but the story should still be able to function without them. This can be a tricky balance; two modern writers whose work features footnotes are Terry Pratchett and Susanna Clarke, and they both use them in different ways.
In Pratchett used his footnotes to insert witty asides to the reader; they can give the impression that Pratchett is telling the story directly to the reader and momentarily digresses with an amusing fact. These asides often show a little bit more of life on Discworld and sometimes even have their own footnotes. Part of the reason they work is because as much as these footnotes add to a story the reader can read an entire Discworld book without the footnotes and it will still make sense and can still be enjoyable. With over forty books in the series it would be easy for Pratchett to have put a footnote on every page about the world, but he doesn’t. Instead he scatters them through his novels and generally keeps them short. It could be possible that Pratchett’s early drafts had more footnotes than his finished ones and he trimmed them during editing to try and find the best effect.
Clarke takes a different approach. Her book, Jonathan Strange and Mr Norrell, is about magic at the being of the nineteenth century in Britain. In Clarke’s world magic is no longer practiced, or so people think, and it has become an academic study. The book is also written in a style that reflects the books of the time in which the story is set, which also used footnotes. In the novel there seems to be a hit of tongue-in-cheek academia about the footnotes. If a magician’s book about magic is referenced Clarke puts it in the footnotes as if it is a real book. She also uses the footnotes to reference intellectual arguments magicians have over a subject, or what one of the books has to say on a subject. Unlike Pratchett’s witty asides in his footnotes Clarke’s more conspicuously suggest academia, witty in their own way too but in a different way to Pratchett’s. Clarke’s also differ from Pratchett’s because while some of the footnotes are very short some appear much longer than Pratchett’s. This may be, in part, because sometimes footnotes in books from the time have very long footnotes, sometimes half a page or more. Once again though the application is strategic.
So, simply because footnotes are associated with academia doesn’t mean that they can’t be used effectively in fiction. If we do use footnotes we have to be aware when we do that we still need to find the balance between telling enough and over telling, as with any other exposition.