Act 3 is full of very meaningful passages depending on what aspect of the play you are interested in analyzing.
For instance, one passage that helps to characterize Juliet further can be found in Scene 2 of Act 3. After Juliet learns that Romeo has just killed her beloved cousin Tybalt, Juliet feels that she has been deceived by Romeo. Not only that, we see Juliet grow up a bit through this passage in that she learns things are not always what they seem. We see Juliet realize that Romeo may not be what he seems to be in the lines,
O serpent heart, hid with a flowering face!...
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb!...
Just opposite to what thou justly seem'st. (III.ii.76-81)
This series of oxymora shows us that Juliet is now seeing Romeo in quite the opposite light.
Juliet grows up even more in this scene when she decides to continue to support and trust her husband, despite what appearances are. We see her realize that she must trust her husband when she proclaims,
Shall I speak ill of him that is my husband?
Ah, poor my lord, what tongue shall smooth thy name
When I, thy three-hours' wife, have mangled it? (III.ii.102-104)
In these lines, Juliet is convincing herself not to think ill of her husband and to defend his honor. She continues in this passage to defend his motive for killing Tybalt. The fact that she is willing to quickly forgive her husband and not think poorly of him, despite not having many valid reasons, and despite appearances, shows us that for the first time in her young life she is learning the true nature of trust and that she is also growing up.
Hence, since both of these passages help to characterize Juliet as growing from her experiences with Romeo, they are both useful passages for analyzing her characterization.