It is interesting to consider what Shakespeare wanted to be most clear about conveying in this Act II, scene ii. And, it is worthwhile to consider what he intended to convey, both theatrically and in terms of the story.
First, he wants the audience to understand that, for most of the scene, Romeo can see Juliet, but she cannot see him. He notices the "light" from her "window" at the beginning of the scene, and when she appears, he describes her actions to the audience, thus making it crystal clear that he sees her.
Juliet, however, when she is interrupted by Romeo's voice, doesn't know who he is ("What man art thou that thus bescreen'd in night/So stumblest on my counsel?") and must discover his identity through listening to his voice:
My ears have not yet drunk a hundred words
Of thy tongue's uttering, yet I know the sound.
Art thou not Romeo. . .?
And so, one of the things Shakespeare is telling us is that Juliet must declare her love to a man she cannot even see. Romeo is in grave danger here in the Capulet balcony, and his remaining in the shadows reminds the audience of the nighttime setting of the scene and the danger of his beeing seen under Juliet's window.
In terms of the story, the scene is telling us how hasty Romeo and Juliet are willing to be. Despite having just met they delcare their love for each other and, upon Juliet's insistence, decide to marry the very next day. Shakespeare provides a nice emphasis on the haste in this scene, as Juliet is under pressure from the Nurse to return inside, even as she challenges Romeo to "send word" the next day "where and what time thou wilt perform the rite."
So, Shakespeare is up to a few significant things in this scene. Theatrically, he wants to make sure the audience realizes that the scene is taking place mostly in the dark and that the stakes of being in this place are, for Romeo, very high. He also wants to demonstrate the absolute haste with which Romeo and Juliet are willing to act -- a haste that will lead to their ultimate downfalls and their deaths.
It is also worth noting that this scene is meant to be played with Romeo and Juliet unable to actually touch each other. Written to be performed in a theater with an "above" or balcony, Juliet and Romeo would have been separated to such a degree that they could not touch. This fact suggests that Shakespeare wanted to convey the desperate desire that the young lovers have to break through the obstacles that separate them to be together -- the obstacle that is their families' hatred being represented by the obstacle of distance in the scene.
For more information on the balcony scene, please follow the links below.