Sense and Sensibility is perhaps Austen's second best known novel, standing as it does in the shadow of Pride and Prejudice. (Project Gutenberg's download numbers support this opinion.) I've watched the Ang Lee version of the movie many times, but had never got around to reading the book. Lee's version of the movie is outstanding, in large part due to the screenplay by Emma Thompson (who also starred). The comparison is fascinating: Thompson's screenplay absolutely captured the spirit of the book, but utterly shredded the text. It was a rare occasion indeed when I found the same words coming out of the mouth of a character in the book as had in the movie.
The story concerns two young women (every Austen story does, although the number varies), Elinor and Marianne Dashwood. Elinor is the oldest, and far more sensible than her younger sister who is governed by her passions. In the first act it's immediately established that the family has suddenly fallen on hard times by the death of the sisters' father, but fairly soon Elinor is courted by the quiet and equally sensible Edward Farrars, while Marianne is courted by the handsome and passionate Willoughby. In the second act, both relationships receive terrible set-backs. And if you're not a fan of Austen but know something of her books, you may be thinking that this sounds a lot like her other books. It's true: every one of them is what could be considered by modern classification as a rom-com. But there's a reason Austen's books have been in print pretty much continuously for 200 years: the woman can WRITE. My favourite line, when Elinor spent some time with one of the more frivolous characters in the book, was "Elinor agreed to it all, for she did not think he deserved the compliment of rational opposition." The beauty of that is how much she says about both characters (and even the state of society as a whole) in one short sentence.
The biggest changes I noticed from Austen to Thompson's screenplay were the third daughter Margaret, and Thompson's swipe at the plight of women during the period:
Elinor: You talk of feeling idle and useless. Imagine how that is compounded when one has no hope and no choice of any occupation whatsoever. Edward: Our circumstances are therefore precisely the same. Elinor: Except that you will inherit your fortune. We cannot even earn ours.
This felt very out of place to me: it may well be accurate, but it's not something Austen would ever have written.
As for Margaret ... the youngest daughter is mentioned perhaps twice in the entire book - and it's not a thin book. I don't think she ever speaks. In the movie she's visible, active, and vocal: Thompson has turned her into the innocent voice of the audience, poking at the mores of the time.