Marianne, by contrast, is the far more emotional of the sisters. Where Elinor may be seen as too sensible, Marianne's fault is that she lacks sensibility. She is far too easily swayed by her feelings and as result makes decisions that have disastrous results (even to the point of putting her own life in peril).
As with all of Austen's novels, the main characters each have a major personality flaw that they must overcome in order to find true love. Such development in character takes time to unfold and so to do justice to the original novel it's important to take time to discover these characters.
Thankfully, the filmmakers took the time to really accurately capture the essence of the novel. We gradually see changes in both heroines as each learns painful lessons from their individual shortcomings and overcomes their mistakes to be able to achieve true love.
The selection of Andrew Davies as the screenwriter was a stroke of genius. With his long list of previous literary adaptations (including the beloved 1995 version of Pride and Prejudice) he seems to understand better than any screenwriter working today what it takes to bring Austen to the screen.
The casting selections were also excellent as Hattie Morahan (Elinor) and Charity Wakefield (Marianne) capture perfectly not only the essence of each of the individual characters but the relationship between the sisters. It also helps that both are fairly close to the age of each of the sisters in the book which is something that was missing from prior adaptations.
The other characters are also well cast. Although the names of the actors and actresses would be unfamiliar to anyone who is not an aficionado of British drama, each brings a high quality performance to the production.
But the real magic to this production is the fact it's the BBC producing this film. The BBC has an affinity for period drama like no other production company. If anyone can adapt Austen and do it justice, they can. No expense was spared in making this production as authentic as possible down to the costumes and filming locations.