Does in Romeo and Juliet have a set rhythm?

Some lines have a set rhythm, some do not. The ones that do, are in blank verse, or just verse. They look like this:

"What, wilt thou wash him from his grave with tears?

An if thou couldst, thou could not make him live.

Therefore have done. Some grief shows much of love;

But much of grief shows still some want of wit"

Note that the first Word of each line is capitalized. This is your hint that this is verse and should be said like this:

What WILT thou WASH him FROM his GRAVE with TEARS?

An IF thou COULDST. thou COULD not MAKE him LIVE.

ThereFORE have DONE. Some GRIEF shows MUCH of LOVE;

But MUCH of GRIEF shows STILL some WANT of WIT.

On the other hand,

"More than Prince of Cats, I can tell you. O, he's the courageous captain of compliments. He fights as you sing pricksong--keeps time, distance, and proportion; he rests his minim rests, one, two, and the third in your bosom! the very butcher of a silk button, a duellist, a duellist! a gentleman of the very first house of the first and second cause. Ah, the immortal passado! the punto reverso! the hay!"

is not in the fixed rhythm of iambic pentameter verse, although it does have its own distinctive rhythm.