A Doll's House is all about maintaining appearances, both for Nora and Torvald; she to keep her secret and he to maintain his position as head of the household. Nora and Torvald mislead each other as she believes that, as long as her intentions are honorable,it is acceptable to have forged her father's signature and he places more importance on his "happy and welcoming" home than any "real" love for his wife.
Nora knows that she is in danger of being discovered, especially as she has been unable to secure Krogstad's position at the bank, despite pleading with Torvald in an attempt to save herself.
When this fails, she pretends that the dance she will do the following evening - the tarantella - is difficult and she clumsily mixes the steps in her desperate attempts to prevent her husband from opening the post box where the letter from Krogstad, revealing her deceit, lies. All her attempts to stop Krogstad, even to the extent of Mrs Linde trying to help, have been unsuccessful. All that Nora is able to persuade Torvald to do is to wait until after the fancy-dress ball before opening the postbox. Torvald believes he is controlling the situation but will allow his wife this small indulgence.
Nora knows she will soon be discovered but, as she believes her husband's love is real, she thinks he will understand. Unfortunately, she will ultimately be unable to "save" herself or her marriage.