Mariana's substitution for Isabella in Angelo's bed (sometimes called the bed trick) has received considerable attention from scholars. Isabella has been sharply criticized for her willingness to allow Mariana to make such a sacrifice. The heroine's purity has been challenged on the basis of her easy compliance with the duke's scheme, which calls for Mariana to commit the very sin which so repulses Isabella. The duke's character has been maligned for the perpetration of this vulgar trick. He is, critics charge, as immoral as the play's corrupt setting. Even the gentle Mariana has been attacked for her role in the deception.
Before making a judgment on the characters or their creator, however, it is important to gain an understanding of the conventions operating on Shakespeare's contemporary audience. When the play was written in 1604, it was customary to have a formal ceremony of betrothal some time before the actual wedding celebration. The betrothal involved repetition of vows and gave conjugal rights to the betrothed. By this custom, it was no more immoral for Angelo and Mariana to share a bed than if they had actually been married.
Claudio and Juliet's secret betrothal, on the other hand, did not carry with it the conjugal rights since it was simply an exchange of promises, not formally witnessed or celebrated. For this reason, Claudio and Juliet are guilty of a crime and immorality, while Mariana's union with Angelo carries with it no stigma.
An awareness of the custom of betrothal casts a new light on the play. Not only does it clear the duke, Isabella, and Mariana of impurity, but it also has the effect of lessening Claudio's crime since there is only a question of a formal public betrothal between crime and convention.
The bed trick is admittedly a contrived bit of dramatic foolery, requiring an audience to believe that a woman can, without discovery, go to bed with a man who knows her and expects another. It further requires that an audience credit the woman's willingness to take part in such a deception after being heartlessly cast off by the man years previously. And finally, the existence of a Mariana who can be Isabella's proxy without smirching her own character is itself an unlikely bit of coincidence.
However, coincidence and the failure of a man to recognize his lover were established conventions of Renaissance drama. The deserted wife's return in disguise to her husband was traditional. Shakespeare's audiences were accustomed to accepting in the theater what they would have scoffed at in real life. The modern reader, then, should bear in mind that the bed trick would not have seemed as extraordinary to Shakespeare's original audience as it does now.
Although contrived, it is certainly necessary. In order to bring the play to its final dramatic conclusion, while maintaining Isabella's virtue, Shakespeare had to devise a way to allow her to refuse Angelo's demands while making him think they had been met. Actual compliance would have stained Isabella's purity, damaging her as a symbol of good and destroying the dramatic effect of virtue set against corruption. A flat refusal would have meant that Claudio's execution would go forward unhindered, bringing the play to a conclusion with no opportunity for repentance, forgiveness, and the application of justice with mercy which together form the play's theme.