First, Juliet is rebellious. We have the obvious example of her marriage without the consent of her parents. It's clear that to her father, Juliet's value is as an asset to cement an alliance through marriage. By marrying Romeo, Juliet defies her father's plans for the sake of her own desires. Further, by marrying the son of her father's enemy, Juliet exposes her family to potential dishonor and ridicule. Thus, she's taking a considerable risk. Her father would disown her if he knew she married Romeo. As is, he threatens to disown her when she rejects the prospect of marrying Paris. It is in this scene, act 3, scene 5 (lines 147–152), when we see Juliet's rebelliousness come to the fore.
Capulet: Is she not proud? Doth she not count her blest,
Unworthy as she is, that we have wrought
So worth a gentleman to be her pride?
Juliet: Not proud you have, but thankful that you have.
Proud can I never be of what I hate...
Teenagers talk back, I know. But in the context of Juliet's position as the daughter of an Italian nobleman during the Middle Ages, to so brazenly defy her father's wishes and to spurn the marriage he has arranged is an extreme act of rebellion. Juliet's family and Verona society expect her to be dutiful, modest, virtuous, and, most of all, silent.
Second, when she chooses to speak, Juliet is candid with her feelings. On numerous occasions, Juliet expresses her feelings for Romeo plainly and without exaggeration. During the balcony scene, Juliet declares her love for Romeo without knowing he is there, but she doesn't retract it once he speaks out.
Juliet: In truth, fair Montague, I am too fond
And therefore thou mayst think my behavior light;
But trust me, gentleman, I'll prove more true
Than those that have more cunning to be strange. (Act 2, scene 2, lines 102–105)
Third, Juliet is impatient. We can attribute this early on to the fact that she is young and sheltered, but as the play goes on, Shakespeare gives the impression that it's part of her character—a heroic flaw that contributes directly to her death. She is the first to suggest marriage and the first to contemplate suicide. Her quick marriage to Romeo is not the best evidence, since Romeo is also in a rush and the two adults in the know, Friar Laurence and the Nurse, fail to prevail on the young lovers to be more careful.
Rather, Juliet's impatience is best revealed in two other scenes. First, her banter with the nurse in act 2, scene 5, lines 31–32: "How art thou out of breath when thou hast breath / To tell me thou are out of breath?" After the nurse comes back from meeting with Romeo to arrange the secret wedding, Juliet literally will not let the nurse catch her breath after walking through the city, so impatient is she to hear about Romeo.
Second, we see Juliet's rashness raised to a fever pitch when she comes to the priest looking to prevent her marriage to Paris in act 4, scene 1, line 67: "Be not so long to speak, I long to die!" Juliet is threatening suicide in front of the priest unless he finds a way to prevent her marriage to Paris. She doesn't balk at the rather far-fetched and drastic scheme of faking her own death, but instead clutches at the sleeping potion like a child trying to snatch away a toy (line 122): Give me, give me! O tell me not of fear!