In act 4, scene 1, Juliet reveals herself to be completely committed to Romeo, despite her father's accelerated plans to marry her off to Paris. She shows indifference to Paris, who tries to exert ownership over her, but she also reveals a capacity for wit when she says she does not own her own face, meaning that it belongs not to Paris, but to Romeo.
Once Paris is gone, Juliet turns to Friar Laurence in deep distress, bringing out a knife and stating that she will kill herself if the friar does not come up with a plan to prevent the marriage with Paris. She says to friar,
I long to die
If what thou speak’st speak not of remedy.
She also shows both courage and commitment to Romeo when the friar warns her that his plan for her to take a potion that will make her seem dead is very risky:
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,
O'ercovered quite with dead men’s rattling bones ...
Juliet means above that she will face any fear, from jumping off a tower to being chained with bears to stay pure and devoted to Romeo.
In act 4, scene 2, Juliet shows her ability to be deceptive for a good cause. She acts now, after formerly having refused to marry Paris, as if she has come around to accepting the marriage, saying to her father,
Pardon [me], I beseech you!
Henceforward I am ever ruled by you.
These two scenes show the public face of Juliet. In act 4, scene 3, we experience some of her interior nature. She is all by herself in the tomb, ready to drink the vial of potion the friar has given, and she feels free to express her fears, stating,
What if it be a poison, which the friar
Subtly hath ministered to have me dead,
Lest in this marriage he should be dishonored
Because he married me before to Romeo?
I fear it is ....
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?
Juliet fears betrayal, wondering if the friar will poison her, and she fears she will suffocate in a closed vault if she wakes too soon before Romeo arrives. She also fears being awake among the bones of the dead. But she shows her courage in nevertheless drinking the potion, despite all her fears, saying she is doing it for Romeo.