The entire play is built on paradox, or contradiction. Two people who are supposed to hate each other to the death instead fall in love. The play pivots on that central paradox and the exploration it inspires of the difference between expectation and reality. Juliet, especially, comments on this paradox. In act 2's balcony scene, she asks why Romeo has to have the name of Romeo [Montague], from her family's hated house, noting that a rose by any other name would still have the same sweet smell. Likewise, when her beloved Romeo kills her beloved cousin Tybalt, she has a moment of anguished paradox, wondering how the one she loves could have murdered the one she loves.
Soliloquy, speech overheard by the audience that reveals a character's inner thoughts, is another dramatic device Shakespeare uses. For example, Juliet's "Gallop apace, you fiery-footed steeds" soliloquy in act 3, scene 2, expresses Juliet's desire for her wedding night and Romeo to arrive faster. This soliloquy sets an amorous mood and highlights the erotic desire the two lovers share. Juliet speaks of Romeo's arrival as
Whiter than new snow upon a raven’s back.
In another soliloquy, in act 4, right before she drinks the potion that will cause her to appear dead, Juliet creates a mood of creepy foreboding as she dwells on death, Tybalt's corpse lying in the same crypt, and her fears of what might go wrong.
Mood is another dramatic device Shakespeare uses, with moods changing between light and dark throughout the play.
Finally, although Shakespeare reveals in the Prologue that the two lovers will come to a bad end, he builds suspense in other ways: we don't know, for example, that Tybalt will kill Mercutio or Romeo, and as the fight is unfolding, the audience is kept guessing. Further, we feel with Juliet her worry about how the plan to avoid marrying Paris will work out. Even if we have seen the play before, we can get so wrapped up in the action that we are anxious about what will happen next.