I'd like to focus for a minute, though, on the conversation between the Dashwood women on the second day of their acquaintance with Willoughby:
"Well, Marianne," said Elinor, as soon as he had left them, "for one morning I think you have done pretty well. You have already ascertained Mr. Willoughby's opinion in almost every matter of importance. You know what he thinks of Cowper and Scott; you are certain of his estimating their beauties as he ought, and you have received every assurance of his admiring Pope no more than is proper. But how is your acquaintance to be long supported, under such extraordinary despatch of every subject for discourse? You will soon have exhausted each favourite topic. Another meeting will suffice to explain his sentiments on picturesque beauty, and second marriages, and then you can have nothing farther to ask."Elinor tweaks her sister about having spent so much time in conversation with Willoughby, and to be honest, she has a fair point, especially when you take into account the conventions of the time.
"Elinor," cried Marianne, "is this fair? is this just? are my ideas so scanty? But I see what you mean. I have been too much at my ease, too happy, too frank. I have erred against every common-place notion of decorum; I have been open and sincere where I ought to have been reserved, spiritless, dull, and deceitful--had I talked only of the weather and the roads, and had I spoken only once in ten minutes, this reproach would have been spared."
"My love," said her mother, "you must not be offended with Elinor--she was only in jest. I should scold her myself, if she were capable of wishing to check the delight of your conversation with our new friend."--Marianne was softened in a moment.
First, their meeting was terribly unconventional, since they were not introduced by a mutual acquaintance, and Willoughy not only made physical contact (rather than simply bowing from a respectable distance), but also made what can only be considered intimate physical contact when he scooped her up and carried her into the house. And she was wearing Regency clothing, probably constructed from muslin. And being wet, the muslin would've been clingy. And possibly a bit sheer. And he would definitely have felt parts of her body that he shouldn't have been in contact with - like her legs. And her ribcage. No wonder Marianne is initially embarrassed about the whole encounter. (I'm entirely serious about this, oddly enough.)
Second, this visit is overly long. A "proper" first visit lasts 15 minutes, half an hour max - rather like the first visit paid to the Dashwoods by Sir John, or the first (awkward) visit by Lady Middleton the next day. Willoughby being there long enough in order to do much more than enquire after Marianne, allow for introductions and exchange the usual sorts of pleasantries that one typically engages in - for instance, an acknowledgement that one enjoys dancing and books is fine, but a detailed discussion of all the various books and authors you enjoy is . . . not. So while Elinor is teasing her sister, there is in undercurrent of disapproval there that causes Marianne to bridle. Well, that and Marianne's general headstrong behavior and self-centeredness.
Notice how quickly Marianne flips on Elinor. She's not only sarcastic; she also says that to behave otherwise than she did would have rendered her "reserved, spiritless, dull and deceitful". Marianne lashes out because, I believe, she knows that Elinor is correct, at least according to the conventions of the time. Marianne ought not to have been so forthright and forthcoming. It was not "proper" conduct. In speaking as she does, Marianne somewhat acknowledges this (she really ought to have been more reserved), but she also attacks Elinor by implication. Since Elinor is calling her conduct into question, thereby showing that she wants to do what is proper, Elinor must be all those things that Marianne says constitute "proper" conduct.
N.B. When Marianne says that she supposes she ought to have talked only of the weather and the roads and to have spoken only once in ten minutes, she is prophetically not far off of how Elinor conducts herself when we reach Volume III, Chapter 12 (or, if you eschew Volume numbering, Chapter 48) and Edward Ferrars pays a call.
It's not too long until Willoughby is hanging out with the Dashwoods all the time, and he and Marianne lapse into easy familiarity - so familiar, in fact, that they behave badly together by speaking ill of their mutual acquaintance. They are, for their time, a demonstration of "how not to act," and it's something we should keep in mind as we read along, because Austen's original readers would have known this quite clearly. Willoughby (aged 25) has obviously secured Marianne's affection - after all, she's completely transparent with her statements, opinions and emotions - so his need to attack the completely nice and proper Colonel Brandon is a hint to us that Willoughby is not, in fact, a truly nice guy. It may also be a bit of foreshadowing vis-à-vis another Willoughby-related plot line that affects Brandon, but first-time readers need only note that Willoughby is behaving badly, and therefore is somewhat "ungentlemanly" in his conduct. And Marianne is an all-too willing participant in the bad behaviour. And Elinor is in the right, and is loyally sticking up for the "sensible" Colonel Brandon - a term that she uses and which we are to understand in the modern sense of the word - he is a man of good sense. (We will learn as the story proceeds that he is also "sensible" in the earlier sense of the word, meaning that he is a man of great feeling, but that does not appear to be Elinor's meaning at this point in time.)