It's widely accepted that Elinor and Marianne, JA's first go at S&S, was epistolatory. But I find it difficult to square this with Ch.23.

In this chapter, we're suddenly and minutely immersed in Elinor's reactions to this gob-smacking news she's just had from Lucy. This is obviously one of the big climaxes in the book (well, come on, the biggest one yet and we're nearly halfway through the novel - that's hardly a spoiler!!!). I can't see any way in which the information presented in this chapter could possibly not have been presented in any conceivable form of this novel. I also can't see how it could have been written in a letter. These are Elinor's inmost thoughts on a subject that, as a basic plot requirement, she's bound to keep secret from everyone, at least up to our Ch.24.