Isabella: And have you nuns no farther privileges? 
Nun: Are not these large enough? 
Isabella: Yes, truly. I speak not as desiring more, 
But rather wishing a more strict restraint 
Upon the sisterhood, the votarists of Saint Clare. (1.4.1–5)

Isabella speaks her first words in the play in this exchange with an unnamed nun, who is preparing her to take orders at a convent. Even as she’s about to pledge vows to strict order, Isabella yearns for even more rigorous “restraint.” On the one hand, her desire for restriction places her in opposition to her brother, whose lack of restraint has led to his arrest. On the other hand, Isabella is strangely like her brother in that they share a tendency to excess. Whereas he is excessive in his pursuit of sensual pleasure, she is excessive in her denial of it.

O, it is excellent 
To have a giant’s strength, but it is tyrannous 
To use it like a giant. 
. . . 
Could great men thunder 
As Jove himself does, Jove would never be quiet, 
For every pelting, petty officer 
Would use his heaven for thunder, 
Nothing but thunder. Merciful heaven, 
Thou rather with thy sharp and sulphurous bolt 
Splits the unwedgeable and gnarlèd oak, 
Than the soft myrtle. But man, proud man, 
Dressed in a little brief authority, 
Most ignorant of what he’s most assured, 
His glassy essence, like an angry ape 
Plays such fantastic tricks before high heaven 
As makes the angels weep, who with our spleens 
Would all themselves laugh mortal. (2.2.135–37, 139–52)

Isabella pronounces these words in her impassioned plea to Angelo. When she first addresses him, Isabella is meek and unemotional. But Lucio, seeing that Angelo will never cave if she remains “too cold” (2.2.62), implores her to passion. As the scene progresses, her language takes on more and more urgency, until she arrives at this moment, when she fully embraces her powers of persuasion. Isabella denounces Angelo’s tyrannical behavior, calling him to an ordinary man who, now “dressed in a little brief authority,” wields his power mercilessly as though he were a tyrannical god. Ironically, Isabella’s withering rebuke turns out to be just the sort of thing that turns Angelo on.

Justice, O royal duke. Vail your regard 
Upon a wronged—I would fain have said, a maid. 
O worthy prince, dishonor not your eye 
By throwing it on any other object 
Till you have heard me in my true complaint 
And given me justice, justice, justice, justice. (5.1.22–27)

In another impassioned plea, Isabella approaches the Duke to accuse Angelo of crimes and call for justice. Isabella gives a remarkable performance here, especially considering that she’s now appearing before a public audience. At the beginning of the play, she was about to pledge herself to life as a nun, which would have meant that she could either speak to men or look at them, but never both at the same time. Now she’s courageously confronting the most powerful men in the city, appealing to one about the sordid crimes of another. It’s worth keeping in mind here that though Isabella has been coached by the Duke for this performance, she only knows him in his disguise as a friar. Thus, when she approaches the Duke here, she doesn’t know that he himself is acting. It’s all the more impressive, then, that she blithely ignores the Duke when he dismisses her as a madwoman and proceeds with her rousing call for “justice, justice, justice, justice.”