This is another of those chapters where Austen doesn't do all that much in order to rewrite this chapter from an epistolary form to a regular novel - I think it's pretty plain with Willoughby's barely interrupted monologue that this was likely originally a letter sent from Willoughby to Elinor, but recast as a face-to-face conversation. Still, even with recasting it as a one-on-one conversation rather than a reading by Elinor with an occasional thought or reaction, I think the characters' discomfort in participating in such a conversation resonates more with the reader, which at least partially explains the justification for moving away from an epistolary format.
Willoughby: I came because I heard Marianne was dying. The servant assures me she's improving, however. Is it true? Is she improving?
Elinor: Yes, but it is SO NOT YOUR PLACE to be here asking questions.
Willoughby: Oh what a rogue and peasant slave am I!
Elinor: (in a repressive tone) You are no Prince Hamlet, Mr. Willoughby. Cut the crap.
Willoughby: I am a victim I tell you! Mrs. Smith was sooooo mean to me, cutting off my inheritance all because I seduced and knocked up a young woman and then refused to marry her.
Willoughby: And THEN I had to go to town and mary that cow, Miss Grey, for whom I don't care sixpence. Not that she loves me, either. She wanted my land, I wanted her money, bada bing, bada boom. Solved my financial problems. But my heart, my heart, my heart has its love. fn1
Elinor: Good God. Are you quoting Heine? How very . . . anachronistic of you. He didn't start writing poetry until years after this novel was published. I insist that you stop it immediately.
Willoughby: Fine. But I still love Marianne and He Who Shall Not Be Named *coughBrandoncough* is a poopypants and I just know she's going to marry him. Willoughby OUT! *flounces off*
fn1 "But my heart, my heart,/My heart has its love" are lines from "The Sea Has Its Pearls", which I translated for National Poetry Month 2009.
You can watch how they played this scene in the 2008 version for the BBC/PBS between the 11 and 14 minute marks. Be sure to stop at 14 minutes if you don't want to see Colonel Brandon & Mrs Dashwood's arrival, etc., since it's also on this segment: