In the play "Romeo and Juliet" by William Shakespeare, the author does show us the strength of Juliet's character - which is remarkable for one so young (she is still only a teen.) However, part of this character strength is her resolve - many people decide to take on a challenging and frightening plan to achieve the reward they want. Fewer people actually have the resolve to carry it through. I think Juliet derives some of her strength from Romeo - two heads/hearts are better than one and we all gain strength and bravado from taking along a friend through a difficult situation. Although Romeo isn't always there in person, Juliet has at last met someone like her, a kindred spirit who will help her stand up for herself.
Earlier in the Act than the above referenced examples, Juliet tells her parents 'No, thank you,' in reference to marrying Paris. It would take a considerable strength and confidence to have said no to parents in that culture. Not only did she say no to her mom to tell her dad, she continued to tell her dad no, and then asked her mom again to tell her dad to at least delay the marriage.
Juliet stood her ground with them. When the Nurse then bailed on Juliet, she still remained on target with her purpose, to stay with Romeo and not marry Paris.
To me, there are a couple of main ways that you can see her doing this.
First of all, she decides to go ahead with the plan that Friar Laurence comes up with. She's going to drink this potion and appear to be dead -- who knows if that will work. She's going to get put in the family tomb and wake up with all these dead bodies. It seems pretty scary.
Second, she shows strength by going to her father and apologizing. Most people have a very hard time apologizing for things and that was one pretty serious fight she had with her dad when she refused to marry Paris. To apologize to him after the things he called her takes strength.