I was thinking again today... Long story made short, my brain is crazy and I ended up explaining everything to myself in footnote format... which got me thinking more (oh my, leave while you can!).

Moving right along, a lot of us have stories about people that are from this world but lots of us only use people from other worlds (I feel like a dufflepud with that one...). The main characters from those worlds often have simple beginnings, but if yours doesn't (or does but you don't want him looking like a fool) you'll probably reference a lot of things that your readers will know nothing about. Glossaries and Appendices are well and good, but sometimes flipping back and forth gets somewhat tiresome. (enter the subject of the thread) Tada! Footnotes! Footnotes are sometimes used in fantasy to translate phrases on the page, but they can do so much more than that. The copy I have of Dracula has footnotes that explain things that explain the slangs or items that were common in the period in which it was written but as a gothic novel, it added a great dimension. Dracula is a gothic novel and so it is written in a journal format. We can't always have people explaining things they know well or take for granted so a foot note is a great way to strength the setting of the scene without taking away from the words themselves. A similar thing was done with pictures in the American Girls series's. You can still keep the glossaries and appendices if you want everything in one place.

*looks up* I went on more than I expected. I warned you this would happen! Good writing, chums.

Hm. Interesting.

I suppose a common danger of footnotes would be to detract from the story -- effectively pulling a reader out of 'the spell' in order to fully explain what is going on.

That's true, yes. If someone is eating a toble melon* you could mention in the scene that they have blue flesh and a hard, dark rind whereas if someone is being compared to one, you might want a foot note that simply mentions that the fruit is known for having a thick shell that is hard to penetrate.

* I just made that up out of thin air, anyone is welcome to it.

Footnotes and such (in fiction, at least) are more often added by the editor of a particular edition than by the author---except in intertextual and "experimental" fiction, where I think they're rather common. But I think they could be a great tool if your POV character would be the sort of person to want to add explanatory footnotes to everything.

That is a good point.

That was awesome, I just might.
When I read Shakespeare, I always get the annotated versions because I don't fully understand all the terms he uses. It is helpful, but 'tis true that it does detract from the "spell."