If I know you well,
You were the Duke’s surveyor, and lost your office
On the complaint o’ th’ tenants. Take good heed
You charge not in your spleen a noble person
And spoil your nobler soul. I say, take heed—
Yes, heartily beseech you.
(1.2.196–201)

During Buckingham’s trial, Wolsey calls one of the duke’s former employees, known as the Surveyor, to give evidence to support the charge of treason. The Surveyor explains that a friar convinced Buckingham that he had a claim to the throne, and that the duke subsequently declared he’d seize the throne if the king died, killing any other claimants if need be. Hearing this, Katherine interrupts the proceedings with the lines quoted here. She draws the court’s attention to the likelihood that the Surveyor, whom Buckingham had fired, can’t be trusted to give faithful testimony. Her observation is very astute, and if the king had acknowledged her point, Buckingham may well have survived. However, the king, under Wolsey’s influence, is convinced of the duke’s treason and sends him to his death.

                I do believe,
Induced by potent circumstances, that
You are mine enemy, and make my challenge
You shall not be my judge; for it is you
Have blown this coal betwixt my lord and me—
Which God’s dew quench! Therefore I say again,
I utterly abhor, yea, from my soul
Refuse you for my judge, whom, yet once more,
I hold my most malicious foe and think not
At all a friend to truth.
(2.4.84–93) 

Throughout the play, Katherine shows herself to be a remarkably self-possessed person who refuses to compromise when her honor is at stake. She therefore speaks out fiercely against the divorce, insisting that the king has no reasonable basis for divorcing her. Not only that, but she refuses to acknowledge Wolsey’s authority as a judge in the divorce proceedings. As she indicates here, she’s aware that the cardinal is the one responsible for “blow[ing] this coal betwixt my lord and me.” His interference in their marriage doesn’t just compromise his ability to evaluate the case impartially, it makes him her enemy. Therefore, she repudiates the cardinal and walks out of her own trial.

I thank you, honest lord. Remember me
In all humility unto his Highness.
Say his long trouble now is passing
Out of this world. Tell him in death I blessed him,
For so I will. 
        . . . When I am dead, good wench,
Let me be used with honor. Strew me over
With maiden flowers, that all the world may know
I was a chaste wife to my grave. Embalm me,
Then lay me forth. Although unqueened, yet like
A queen and daughter to a king inter me.
I can no more.
(4.2.182–86, 189–95) 

These moving lines are the last Katherine speaks in the play. She has grown ill and now prepares for her death. The passage begins with her bidding the Spanish ambassador Capuchius goodbye, asking him to convey her enduring love and respect to Henry. She then turns to her two attendants, Griffith and Patience, and gives instructions for how to prepare her body once she’s passed. She stresses that her body should be strewn with “maiden flowers” to express that she remained “a chaste wife” to the very end. Even though she has been “unqueened,” Katherine retains her nobility of spirit and faces death with her head held high.