"Costumes are also used to show Mr. Darcy’s evolution as he comes to love Elizabeth Bennet and let go of his snobbery. His costume had a series of stages. The first time we see him he’s at Meryton, where he has a very stiffly tailored jacket on, and he’s quite contained and rigid. He stays in that rigid form for the first part of the film.
By the time we get to the proposal that goes wrong in the rain, we move to a similar cut, but a much softer fabric. And then later he’s got a completely different cut of coat, not interlined, and he wears it undone.
The nth degree is him walking through the mist in the morning, completely undressed by 18th-century standards. It’s absolutely unlikely, but then Lizzie’s in her nightie, so what can you say?”
(Jacqueline Durran, Costume designer)