Social status is arguably the most important characterization tool in A Doll's House. Most every character is strictly bound into the roles that society places them in. Nora must be the dutiful housewife. Christine only knows how to be happy if she's fulfilling the same role. Torvald must be the dominant husband. Krogstad struggles against the negative perception the community has of him. For the vast majority of the characters, social status is the defining feature. It is when the characters struggle against these roles that the play's main conflicts are ignited.
As in most drama, the character's actions define them. Krogstad spends most of his time threatening and blackmailing, showing us what a nasty sort of person he can be. When he does the whole switch-a-roo at the end and releases the Helmers from his clutches, we see that he's capable of compassion. (He just needed to be loved. Aww, how sweet.)
Christine tries her best to help Nora throughout the play, which gives us an idea of her caring nature. She thrives on helping people. We see this again in her union with Krogstad. She's not always warm and squishy though. When she hangs Nora out to dry at the end, by forcing Nora to confront Torvald, we see that that this motherly figure isn't afraid of some tough love.
Then there's Torvald. His actions paint him as overbearing and superficial. He's the archetypal overbearing husband. His little concessions to Nora show, though, that he does love her in his way. Also, for most of the play, he seems like an upstanding citizen, even if he is kind of a jerk. In the end, however, when says he'll give in to Krogstad's wishes, we see that it's just the appearance of things that he's concerned with, not the morality behind it.
Lastly we have Nora. Her doomed struggle to keep her secret is the central action that drives the play. Her underhanded methods tell us a lot about who she is as a character. We see what society has forced her to resort to in her desperation. In the end, however, Nora allows Torvald to know the truth. Then she deserts him. The old Nora would never have done these things. The change in her character is all too clear through these final actions. She won't be anyone's doll any longer.
If you examine the names of the two couples in the play, some pretty interesting ideas are unearthed. Take Christine for instance. Notice that it's Christ-tine. Mrs. Linde has definitely lived a Christ-like life of self sacrifice. Then there's Krogstad. His name comes from the Old Norwegian word for crooked, we certainly see him doing a lot of crooked things over the course of the play. Is he perhaps the Satan to Christine's Jesus?
Now we come to Torvald and Nora. Both of their names have pagan origins. Torvald = Thor, Norse god of thunder; Nora = Elenora or Helen. Throughout the play, we see the Helmers engaging in pagan-like activities. There's the Christmas or Yule tree, which was originally a pagan thing before Christians adopted it. Then, of course, there's the feasting, dancing, and celebrating, which all go along with the old pagan celebration of Yule.
Krogstad and Christine are in a way the "Christian" couple, while Nora and Torvald are the "pagan" pair. That's not to say that the Helmers are literally pagan, it just means that, in some ways, their lifestyle resembles a sort of pagan insouciance. Krogstad and Christine have both lived lives of hard work and sacrifice. These are some of the major tenants of Christianity. The Helmers, on the other hand, seem to live lives of innocent celebration. By the end of the play, it seems that the "Christian" couple is rewarded for their lives of toil and sacrifice. Nora and Torvald, for their part, are awakened like Adam and Eve in the Garden of Eden. Their innocence is shattered. When Nora walks out of the house she's only just beginning to toil.