I love to use this speech as an example of complex language: "the mask of night is on my face" is complex language for "it's dark". Juliet is saying, "you know it's dark, or you would see me blushing right now", but she doesn't. The poetry is as heightened as the circumstance; the language is complex. An example of simple language: Viola asks, "What country, friends, is this?" (1.2.1). There is nothing complex about that; we certainly don't need to grab our lexicons to decipher what Viola is saying.
The First Folio printing lists line 99 (897 in the Folio) as 'behaviour' rather than the truncated 'haviour' that often appears in modern editions, and I much prefer this syntax. I like the fact that the line becomes an Alexandrine (12 beats), that it's not tidy. There is nothing tidy about what Juliet is going through at this particular moment, and I think modern editors do actors a disservice (and audiences, too) by trying to make the language fit within the regularized structure of iambic pentameter. Things are moving at light speed. Juliet's just been caught professing her love by the very object of her affection. What young lover would be rational enough to simplify her explanation in such circumstances? I know I'm not alone in admitting that I certainly wouldn't be so composed or collected.
Thou know'st the mask of night is on my face,
Else would a maiden blush bepaint my cheek
For that which thou hast heard me speak to-night
Fain would I dwell on form, fain, fain deny
What I have spoke: but farewell compliment!
Dost thou love me? I know thou wilt say 'Ay,'
And I will take thy word: yet if thou swear'st,
Thou mayst prove false; at lovers' perjuries
Then say, Jove laughs. O gentle Romeo,
If thou dost love, pronounce it faithfully:
Or if thou think'st I am too quickly won,
I'll frown and be perverse an say thee nay,
So thou wilt woo; but else, not for the world.
In truth, fair Montague, I am too fond,
And therefore thou mayst think my 'havior light:
But trust me, gentleman, I'll prove more true
Than those that have more cunning to be strange.
I should have been more strange, I must confess,
But that thou overheard'st, ere I was ware,
My true love's passion: therefore pardon me,
And not impute this yielding to light love,
Which the dark night hath so discovered.