Everything in Act IV demonstrates the desparation people feel when things happen that are out of their control.

Juliet is going to be married to Paris and she doesn't want to, so she goes the the friar ready to kill herself. He notes her desparation with the word desperate:

Which craves as desperate an execution.
As that is desperate which we would prevent.

He says this having an idea about a solution to her problem. Juliet then uses hyperbole to model what great lengths she would go to in order to ensure their marriage doesn't happen:

O, bid me leap, rather than marry Paris,
From off the battlements of yonder tower;
Or walk in thievish ways; or bid me lurk
Where serpents are; chain me with roaring bears;
Or shut me nightly in a charnel-house,

She'd jump from a tall building, or go get locked in a tomb, or chain herself up with roaring bears...

Later, upon hearing Juliet's fake confession, Capulet hurries to get the wedding all ready. It's almost as if he wants to get it done before she changes her mind and they go into a fight again.

Next, Juliet has her scene taking the potion. Desperation is further demonstrated in this scene because she takes the action of laying down a dagger just in case the potion doesn't work. She has a preventative waiting right there. She asks several rhetorical about what different ways the potion might not work.

What if it be a poison, which the friar
Subtly hath minister'd to have me dead,
Lest in this marriage he should be dishonour'd,
Because he married me before to Romeo?
I fear it is: and yet, methinks, it should not,
For he hath still been tried a holy man.
How if, when I am laid into the tomb,
I wake before the time that Romeo
Come to redeem me? there's a fearful point!
Shall I not, then, be stifled in the vault,
To whose foul mouth no healthsome air breathes in,
And there die strangled ere my Romeo comes?
Or, if I live, is it not very like,
The horrible conceit of death and night,
Together with the terror of the place,--
As in a vault, an ancient receptacle,
Where, for these many hundred years, the bones
Of all my buried ancestors are packed:
Where bloody Tybalt, yet but green in earth,
Lies festering in his shroud; where, as they say,
At some hours in the night spirits resort;--
Alack, alack, is it not like that I,
So early waking, what with loathsome smells,
And shrieks like mandrakes' torn out of the earth,
That living mortals, hearing them, run mad:--
O, if I wake, shall I not be distraught,
Environed with all these hideous fears?
And madly play with my forefather's joints?
And pluck the mangled Tybalt from his shroud?
And, in this rage, with some great kinsman's bone,
As with a club, dash out my desperate brains?