First, you have to get the casting exactly right. Jane must not, above all, be too pretty. This is more important than the age of the actress playing her. To me, the most successful Janes have been the most unremarkable upon first glance. As viewers, we have to be able to project a bit of our own outrage at the adults who insist on imbuing Jane's already unfortunate childhood with their own emotional and religious terror. Though prettiness is thoroughly punished in this story, we should not miss it. After all, in one of her best moments Jane insists: “I am no bird; and no net ensnares me; I am a free human being with an independent will."
Then, of course, there's Rochester. Oh, Rochester. As one who has never fallen for Mr. Darcy or any other Austen men, I thought I would be immune also to Edward Rochester. But there is something so deliciously cruel in his charisma, especially when stacked against Jane's moral strength, that makes me long for his redemption. His suffering is second only to poor Bertha's (locked in the attic all those years; you wonder if he ever went up for a visit...) and any actor looking to take on this role must be able to juggle anguish, sly humor and a barely-visible heart of gold. I'm partial to how Orson Welles played him in 1943.
My two other most important characters to get right are Helen and St. John. Jane is met with many interpretations of Christian morality throughout the story, and these two have equally pious but otherwise very different personalities. Dear, sweet Helen could easily turn into a big-eyed, saccharine parody of a sickly orphan. But Charlotte Bronte writes her as one of the few rays of hope in Jane's early life at Lowood School, where they both endure the puritanical tyranny of Mr. Brocklehurst. Helen has a quiet intelligence and confidence, credit for which she attributes modestly and entirely to her faith, but Jane, who is more skeptical of “heaven and the universal parent,” loves Helen for her humanity.
St. John Rivers, the repressed missionary whom Jane stays with after fleeing Rochester and his clandestine marriage, is often cast rather two-dimensionally. He should not be portrayed as the villain trying to guilt-trip Jane into giving up her love for Rochester; he is really an uptight but good-intentioned friend whose proposal helps Jane realize that she is far too passionate for missionary work.
Finally, there's Bertha, the infamous Madwoman in the Attic. Granted, her character arc in the novel begins spectrally and ends with an act of suicidal revenge, but the few clues we get from her brother's visit elicit curiosity about her origins. More than any other character, Bertha is rooted in 1847; whatever her “madness” is, there is no treatment; whatever her life could be outside the cold walls of Thornfield is irrelevant because Rochester can't divorce her. Ever since I read Wide Sargasso Sea by Jean Rhys (a brilliant, nightmarish exploration of the Caribbean meeting between Bertha and Rochester), I've wanted Bertha to be more sympathetic in the movies.
But this story belongs to Jane, and she will have many readers yet, and many incarnations on the screen.