In Shakespeare's Romeo and Juliet, why does the Nurse speak in verse? She's not a noble character.
Shakespeare did not use verse only for noble characters. His choice to use prose or verse was sophisticated and purposeful.
To start, there were two types of "speech" Shakespeare used. First, he would use prose—everyday, ordinary speech, or, he would use verse—often rhyming. ("Rhyming" means the word at the end of one line would sound the same as a word at the end of a line before or after...sometimes separated by another line ending with a different sound.)
Note the rhyme in the first four lines of Sonnet 29 (a beautiful love sonnet you should read if you get the chance).
When, in disgrace with Fortune and men's eyes,
I all alone beweep my outcast state,
And trouble deaf heaven with my bootless cries,
And look upon myself, and curse my fate...
In the first and third lines, "eyes" and "cries" rhyme; in the second and fourth lines, "state" and "fate" both rhyme.
The use of rhyming makes speech sound very musical. Shakespeare would not use rhyme, for instance, for any character either insane or pretending to be insane; he would use prose instead...
...whenever verse would seem bizarre: in serious letters (Macbeth to Lady Macbeth; Hamlet to Horatio), in proclamations, and in the speeches of characters actually or pretending to be mad (Lady Macbeth; Hamlet and Ophelia; Edgar and King Lear) -- verse is apparently too regular and orderly for expressing madness.
There are several other areas in which prose is used by Shakespeare: usually in more serious situations, such as...
...cynical commentary...reducing flowery speech to common sense terms; when the rational is contrasted with the emotional...and for scenes of every day life—to name a few.
Shakespeare's comedies jump between prose and verse:
Prose and verse interlink, interlock, and interinanimate each other...often and...densely in Shakespeare's comedies...
Prose will be used effectively when a scene is changing, but also when someone is angry. Perhaps we could note that prose is used on a more serious note, while verse can be comic, but also works as a structural support running through the play.
…prose is used, much oftener than verse, as the medium for verbal hijinks, verbal fireworks, and verbal filigree, while verse serves more often as the vehicle for the nuts and bolts with which the actions and the passions of the plot are put together.
With this said, we can acknowledge that verse was often used in Shakespeare's comedies—and, it would seem, when comic relief was used. Comic relief is used by Shakespeare for a specific purpose:
Comic relief usually means a releasing of emotional or other tension resulting from a comic episode interposed in the midst of serious or tragic elements in a drama.
The Nurse is a source of comic relief in the play. Tragic as the events that take place are, the Nurse's foolishness breaks up some of the tension present.
...it is NOT ACCURATE to say that "the lower classes speak prose and the upper classes speak verse." ...when the lower classes figure in serious or romantic situations, they may speak verse...
...is often interpreted as a comic foil to Juliet.
In Act Two, the Nurse provides comic relief—Mercutio makes fun of her. She and her servant, Peter, arrive—he is carrying her excess of clothing. Mercutio pokes fun comparing her to, "A sail, a sail!" (II.iv.93) and saying her "fan's the fairer face" (98-99), but she will "bite" back.
As a comic figure, the Nurse at times speaks in verse.