Acting does matter in character dramas.

A movies and TV series are different from books in that with a book there's just one person responsible for everything, the author while in a movie or a TV show there are scores of people the ultimate success of the production depends on. You may have a good script for instance but if the director sucks the result will be mediocre at best. On the other hand even the most brilliant director can't do much if the cast are inadequate. A poor cast was what ultimately ruined the 1979 BBC adaptation of P&P, which had a very decent script.
In 1995 the BBC finally managed to pull it off with Pride and Prejudice, and to a great extent this success can be attributed to the brilliant acting. Practically every actor did their job surprisingly well. It probably has to be said at this point that BBC productions had generally been notorious for their embarrassing amateurishness. The 1995 P&P adaptation boldly broke with this dubious decades-old tradition. And the difference is most noticeable in the acting, it's as if the BBC finally decided to stop casting amateurs and invite professional actors instead. Colin Firth is the most convincing Darcy I've seen, Jeniffer Ehle as Elizabeth delivers an equally solid performance. Mrs. Bennett is totally outstanding in this production and Mr. Collins is perhaps once again the best I've seen so far.
There is no doubt that in a character drama of the sort that Pride and Prejudice is an example of casting is paramount and obviously the producers of this adaptation had a profound appreciation of this fact, making some very good choices for the most important characters.
It's a job well done.