What parts of Romeo and Juliet show passion to be unpredictable? Please provide quotes.

The passion with which Mercutio spins his tale of Queen Mab could be considered unpredictable. At first it seems as though he is just making up a funny story about a fairy who makes men dream. The audience may think Mercutio is simply trying to amuse his friends, but then his storytelling takes a dark turn as he delves into violence and sexual innuendo. The story is initially harmless and light hearted:

She is the fairies’ midwife, and she comes
In shape no bigger than an agate stone
On the forefinger of an alderman

From line 58 to line 86 of Act I, Scene 4, Mercutio's speech is without tension but then an unpredictable passion seems to overtake him at line 87. What had been a story about a fairy and lovers takes on an edginess with references to "cutting foreign throats" and then to brutal sexuality in the last few lines:

This is the hag, when maids lie on their backs,
That presses them and learns them first to bear,
Making them women of good carriage.

Mercutio displays more emotion at the end of his speech and works himself into quite a fit, so much so that Romeo has to quiet him:

Peace, peace, Mercutio, peace.
Thou talk’st of nothing.

Another possible example of unpredictable passion occurs in Act III, Scene 5 when Lord Capulet totally changes his mind about Juliet marrying Count Paris and bursts into a rage when his daughter goes against him. Prior to this scene, Capulet had shown a good deal of humility and common sense. When Paris first asks to marry Juliet, Capulet thinks only of his daughter's welfare, telling Paris to first win Juliet's love. Again in Act I, Scene 5 he is the voice of reason when Tybalt wants to fight Romeo during the party. Capulet even speaks well of Romeo:

Verona brags of him 

To be a virtuous and well-governed youth).

In Act III, however, he displays a much different attitude. When Lady Capulet informs him that Juliet is not willing to marry Paris he goes berserk, showing a passion that the audience had not seen before:

How, how, how, how? Chopped logic? What is this?
“Proud,” and “I thank you,” and “I thank you not,”
And yet “not proud”? Mistress minion you,
Thank me no thankings, nor proud me no prouds,
But fettle your fine joints ’gainst Thursday next
To go with Paris to Saint Peter’s Church,
Or I will drag thee on a hurdle thither.
Out, you green-sickness carrion! Out, you baggage!
You tallow face!

Capulet eventually threatens to disown Juliet and kick her out of the house. This is a different man from the one who urged the Count to "woo her, gentle Paris, get her heart."

Other displays of unbridled passion are predictable. Tybalt's aggressiveness, Romeo's infatuation with Rosaline and then Juliet, Mercutio's reaction to Tybalt's insults and even Juliet's impatience with the Nurse seem normal reactions given the circumstances.