The comic character of the Nurse displays a fickleness at certain points in Shakespeare's play that defies explanation, especially in Act IV, Scene 5 when she urges Juliet to marry Paris although she knows that Juliet has already married Romeo. So, after she returns from suffering the jests of Mercutio and then speaking with Romeo, the Nurse takes advantage of her position of holding important knowledge that Juliet desperately wants to hear, perhaps, again because of her fickle nature. Also, she may well be enjoying the opportunity to be the center of Juliet's attention, complaining of her back, and scolding her,
....Ah, my back, my back!
Beshrew your heart for sending me about,
To catch my death with jouncing up and down!
The Nurse continues to tease the anxious Juliet, an indication that she is close to the maiden.
In this pivotal scene, Shakespeare achieves both low humor (from the Nurse) and high suspense (from Juliet).
We first meet the nurse in Act 1, Scene 3 when Lady Capulet asks her to summon Juliet. It becomes obvious that Lady Capulet has some important issue to discuss with her daughter. We can correctly assume that the nurse is aware of what the subject of Lady Capulet's conversation is to be. Throughout Lady Capulet's talk with Juliet the nurse repeatedly interjects, thus delaying the point that Juliet's mother wants to make. Even when Lady Capulet asks her to stop talking she insists on rendering her opinions.
On another occasion, in Act 2, Scene 4, she indulges in a long conversation with Romeo and does not even tell him what message Juliet has given her. She expresses her disdain with Mercutio's lewd remarks about her and then goes into a lengthy advisory talk about how Romeo has to treat Juliet and how she expects him not to mislead the young lady. She tells him about Paris wooing Juliet and then alludes to fidelity by asking if Romeo's name does not also begin with an 'r' like rosemary. Rosemary in this instance, is a reference to a herb that is associated with fidelity.
In another instance, in Scene 5 of the same Act, she keeps Juliet on tenterhooks before finally imparting Romeo's message that she should go to Friar Laurence's cell where they would be married.
In all these instances, Shakespeare uses the nurse as a secondary narrator and adviser. In Act 1, for example, she provides details about her relationship with Juliet and constantly espouses homilies. She does the same in the other two instances mentioned above. She seems to believe that her advice is needed and she provides it freely. Shakespeare, in this sense, uses her as the voice of common-sense because her warnings and suggestions have a reasonably basic logic about them.
Furthermore, since the nurse is a servant in the Capulet household, her delaying tactics are an opportunity for her to assert some authority. She uses these long-winded conversations to add some status to her otherwise lowly position. She can expound and philosophize on a very basic level and share her street smarts with the upper crust of society and they, she probably assumes, will respect her for that because what she says does, after all, make sense.
In addition to this, the nurse also cares much about Juliet. Shakespeare establishes her as an important figure in Juliet's life by allowing her to provide so much intimate detail about her association with the naive young Capulet. It becomes quite clear that the two share a very close relationship. The nurse is obviously afraid of losing the young maiden's company and would, if she could, delay Juliet's departure (through marriage) for as long as she can.
It is also quite apparent that Shakespeare uses the nurse's long speeches to add humor to the proceedings. Although she is not an entirely clownish character, the nurse's clumsy and sometimes inappropriate remarks do provide much mirth. The humor, at times, also adds some tension, such as when she takes so long to tell Juliet what Romeo's message is. The clearly anxious girl becomes somewhat impatient with her well-meaning assistant when she takes so long to get to the point. The audience is also moved, surely, to become quite irritated with the character and wish for her to move on.
All in all, Shakespeare's technique adds to the greater enjoyment of the play and helps the audience engage with the unfolding events. Because the majority of his audience most certainly must have been from the same or a similar social and economic background as the nurse, they would have derived great enjoyment from watching her since they could identify with her character.