This scene is particularly powerful because Othello has become violently angry with Desdemona, taking Desdemona completely by surprise. In the previous scene, Othello slapped Desdemona in front of Lodovico. She has no idea why she is being treated this way. In Act 4, scene 2, Othello treats her like a harlot, calling her whore, accusing her of being false, and weeping as he makes these accusations. Desdemona can hardly defend herself because Othello has named no names. He has yet to mention the fact that he thinks she is sleeping with Cassio. She can only repeat that she is his true and faithful wife. It is not Desdemona's nature to return anger with anger. And these accusations are so surprising, so unlike the way Othello has treated her in the past, that she reacts with disbelief, almost trance-like. She tells Emilia that she is half-asleep, and that she no longer has a lord (or husband). For Othello has changed.
I don't think Desdemona is necessarily passive. She goes to Iago to find out why Othello is angry, and believes Iago when he tells her that some "business of the state" is the cause. She can only hope that this fit of anger is temporary and that Othello will once more become the man she married. The idea that Othello is plotting to kill her has never crossed her mind.
Desdemona is devoted to her husband and is innocent and naive as to the complexity of human feelings and relationships. We see that she is not worldly when she is in conversation with Emilia in Act IV scene III she reveals the depth of her piety. When Emilia reveals that she would
make her husband a cuckold to
make him a monarch
Desdemona is shocked. She believes that the truth will always be revealed and that even her misguided husband will see what is right and true. Desdemona feels as a loyal wife she is subject to the will of her husband, as she was formerly subject to the will of her father. It is not her place to challege her husband in any context. She does appeal to him to let her live, but his physical and mental strength are too much for her.
Desdemona is an innocent. She has been talking with Casio about helping him with a situation by advocating on his behalf with Othello. She has no idea that Othello has been set up to believe that she has had relations with Casio. When she talks with Othello in their marriage bed he only perceives images of her with Casio.
In Act V scene II Desmonda and Othello are beginning to re-bond when she brings up Casio's name. Othello confronts Desmonda about the handkerchief that Casio had of hers. She panics and lies about it. Desmonda continues to have trust in Othello and does not know that he has decided to kill her because he believes she has been unfaithful. She does not act as harshly in her defense because of her trust in their love.