Can anyone explain in layman's terms how this personality training works?

Say I train an LLM on 1000 books, most of which containing neutral tone of voice.

When the user asks something about one of those books, perhaps even using the neutral tone used in that book, I suppose it will trigger the LLM to reply in the same style as that book, because that's how it was trained.

So how do you make an LLM reply in a different style?

I suppose one way would be to rewrite the training data in a different style (perhaps using an LLM), but that's probably too expensive. Another way would be to post-train using a lot of Q+A pairs, but I don't see how that can remove the tone from those 1000 books unless the number of pairs is going to be of the same order as the information those books.

So how is this done?