"Last night you began to tell me strange things about my ring, Gandalf," said Frodo. "And then you stopped, because you said that it was getting late, and we still had another sixteen pages to go..."
Well, nearly!
Tolkien loves this technique, as well he might, being no little of a loremaster himself. Lord of the Rings is full of the beggars. Gandalf, Elrond, Tom Bombadil, Aragorn, Galadriel, Treebeard - and Faramir and Bilbo and Merry and Frodo himself, on a lesser scale - all serve this function at some point or another. Loremaster exposition is one of the opposed methods to maid-and-butler dialogue/As You Know, Bob: it allows one character, who knows huge dollops of stuff almost nobody else knows, to helpfully inform the reader in the process of reasonably informing the character.
For my present purposes, the main use of a loremaster is to tell a story, or the selected highlights of a story, which neither their real nor his fictional audience could otherwise be expected to know. Because they are so lore-wise, they potentially have a lot of such stories at their fingertips.
( Loremasterly exposition in Tolkien, and its higher mode's aspirations to cover current and future narratives. )
On re-reading Chapter 2, The Shadow of the Past, I find whole new levels of craft in the way Tolkien breaks down this massive infodump, sets it to a compelling rhythm, and controls its tone for fascination, tension, oppression, and release. The only reason I'm not going to analyse it right here and now is a practical one - it's not very close to what I'm trying to achieve. It would be truer to say, in fact, that several features of Three Katherines are a reaction against what it represents. Let's turn now to what I hope to get out of this.
( Survey of loremaster exposition opportunities in Killer-Kate and Luke Lackland. )
So I don't get to do much exposition of secondary stories by loremaster at all in Three Katherines - and what I do, is not likely to come from the mouths I expected. My takeaway is to focus on Elegant Elder Sister where that needs doing, and to a lesser and highly specialized extent on: Shiny Lurker, Hero-Father, and Mostly Okay Genius. Hm-m-m!