For my money, Juliet is practical, brave, and headstrong. Here are some quotes to get you started on any one of these characterizations.
In the opening of the balcony scene, she displays her practical mind when, as Romeo stands below her window telling the audience how much he loves her, she is considering the huge obstacle to that love -- he is a Montague and her sworn enemy:
O Romeo, Romeo, wherefore art thou Romeo?
Deny thy father and refuse thy name.
Or, if thou wilt not, be but sworn my love
And I'll no longer be a Capulet.
Juliet's practical turn of mind is revealed in her first words in the famous balcony scene. She speaks not about how dreamy-eyed in love she is, but rather the practical events that must happen in order for them to be together.
A good example of her bravery comes quite late in the play. She has just been kicked out of her home by her father (if she refuses to marry Paris) and had her closest confidant, The Nurse, tell her she should marry Paris and forget Romeo. She decides to go completely alone to the Friar and beg him for remedy. If he has none, she vows to kill herself or undergo any other gruesome act, rather than marry Paris:
O, bid me leap, rather than marry Paris,
From off the battlements of any tower
Or walk in thievish ways, or bid me lurk
Where serpents are. Chain me with roaring bears,
Or hide me nightly in a charnel-house.
Ultimately, she commits a huge act of bravery, when she drinks the potion that the Friar gives her, not knowing if it will kill her or merely cause her to seem dead.
Her headstrong nature can actually be seen in the events I have described above, and in her determination to marry Romeo, even though he is her family's enemy. She is the one that pushes for marriage, and one wonders if Romeo would have had the gumption without her headstrong presence behind him.
Naturally, I think that your impression of Juliet is going to be critical in forming your writing about her character. In the end, it's what you feel about Juliet that is going to drive this assignment. You certainly can collect what others say about it, but your perception of her is critical. With that in mind, I think that you can make a case for Juliet being really quite resourceful and quite intelligent, perhaps even more than Romeo. Juliet is savvy enough to understand that as a young woman in a patriarchial setting, she is not able to give into the Romantic flights of fancy in which Romeo can indulge. In their first meeting, she speaks about love with an experience and insight that belies her age and is coy with the nurse in trying to find out Romeo's name. When their love continues, Juliet is quite direct in asking Romeo if he loves her. This is fairly bold and intelligent for a character who is seen as a "star crossed lover." I think that being able to come up with the plan for them to escape would be another instance of her intelligence. These moments might be able to give you specific lines where her intelligence can be explored as a summation of her personality.
Thank you! This has help greatly