Both caesura and enjambment serve to capture Juliet's emotional state as she digests the news that her newly wed husband has just killed her beloved cousin.
Caesura is known as a pause in the middle of a line of poetry. It can be created with punctuation in the middle of the line, or it can simply be created as a natural pause in the words. Juliet's lines 76-88 in Act 3, Scene 5 contain several instances of caesura. We can especially see caesura in the string of oxymora Juliet uses to describe Romeo. When we hear the pauses between elements of the oxymora, we can feel Juliet's mind working to grasp the conflicting ways she now views Romeo and to grasp the idea that she feels she can no longer trust him. We especially see caesura in the lines:
O serpent heart, hid with a flow'ring face!
Beautiful tyrant! fiend angelical!
Dove-feather'd raven! wolvish-ravening lamb! (76-79)
In the first line, the comma between "heart" and "hid" creates caesura, while the exclamation points create caesura in the other two lines. The pauses created by the punctuation serve to emphasize the contrast between the images in the lines. They also serve to emphasize Juliet's emotions as she now realizes that what she thought was beautiful and wonderful can now actually be evil and deceptive.
Enjambment is the opposite of caesura. Through the use of enjambment, a person reads one line of poetry into the next without any stops. Instead of the reader's eyes stopping at either the middle or the end of line, the reader's eyes are automatically carried down into the next line because there are no stops. The effect can produce a sense of speed, which when capturing emotions, can create the sense of rambling thoughts that one can think in times of intense emotion or stress. When one is faced with an earth shattering realization, such as Juliet is being faced with, our thoughts sometimes blend into one big, messy whole. Shakespeare's use of enjambment in this passage creates just such an effect. We can especially see enjambment in the lines:
O nature, what hadst thou to do in hell
When thou didst bower the spirit of a fiend
In mortal paradise of such sweet flesh? (83-85)
Since there are no punctuation marks in these lines to force a reader to stop, a person reads one line into the next at rapid speed. The speed at which we read these lines shows us that this is a question Juliet feels she absolutely must ask. It does not make sense to her that her God, or nature, would create someone who looks as beautiful as Romeo who could also do such evil deeds. Through reading these lines created with enjambment, the reader better sees just how much the issue of Tybalt's death is weighing on Juliet's mind and soul.