Ibsen's play, A Doll House, is structured somewhat differently than dramas based upon the form of the Greek and Roman plays, copied by many playwrights—including Shakespeare. Instead of having the standard five-act format used by earlier dramatists, Ibsen placed the action of his plays into three acts.
Whereas the five-act play used each act for a specific purpose, Ibsen's acts combine these purposes. The five-act play was structured as follows:
The five acts denote the structure of dramatic action; they are exposition, complication, climax, falling action, and catastrophe.
However, when Ibsen wrote A Doll's House, he fused exposition and complication together—introducing Nora and other major characters and their personalities, interactions, etc.—and joined this exposition with the complication in the form of Krogstad, Nora's illegal actions, and his threats to expose her—all in the first act. The climax occurs in the second act: this is when Krogstad leaves the letter exposing Nora's actions in the mailbox, awaiting Torvald's return. In the third act, the falling action and "the catastrophe" include the reconciliation of Kristine and Krogstad, Krogstad's change of heart, the impending death of Dr. Rank, Torvald's awareness of what Nora has done (along with his relief in learning Krogstad will not pursue the matter), and Nora's realization that she has been living a lie, and married—bearing children—to a man she does not know. [While many theater-goers were aghast at Nora's departure at the end (and Ibsen had to write an alternate ending for some performances), the play was in great demand throughout Europe.]
The dialogue, very different from the work of other writers, like Shakespeare, is delivered in prose. There is stage direction included. And unlike so many plays before, there is no "interior dialogue"—there are no soliloquies or asides, for example. We do not learn about character and what they are thinking through the traditional means of speaking to the audience, but through the interaction, discussion and actions between the characters on stage. Whereas Shakespeare's characters spoke to their audiences, Ibsen's characters do not.
For example, when Nora is worried about something, she does not address the audience, but thinks aloud to herself or pours out her concerns to Kristine Linde, one of the major reasons for Kristine's presence in the play—as is also the case with Dr. Rank, who Nora also speaks openly to...to an extent.
We see this when Nora believes that she will be exposed and is afraid that Torvald will take the blame for what she has done: this is, as she explains to Kristine, what she calls "the miracle"—
But it's so awful, Kristine. It mustn't take place, not for anything in the world.
When Kristine leaves the room, Nora (in speaking aloud to herself) says that she has only a short time to live—inferring that she will commit suicide rather than have Torvald take the blame for what she has done. (It's not a moral sin, but a "social" sin.) Ibsen writes:
… [...she looks at her watch.] Five o'clock. Seven hours till midnight; and then four-and-twenty hours till the next midnight. Then the Tarantella will be over. Twenty-four and seven? Thirty-one hours to live.
Ibsen's play was controversial in many ways, and unusual in its structure as well.