What main techniques and language devices are used in Act 1, Scene 5 when Romeo and Juliet first speak to each other?

One of the key aspects that Shakespeare uses in this scene to heighten the action is dramatic irony, for we in the audience know the identities of the lovers and that they are from feuding households and the barriers their love will face. The action that immediately preceeds the Nurse's revealing of the identities of the lovers allows us to see their attraction and falling in love with each other.

However, crucially, the dialogue makes it clear that for both Romeo and Juliet, the revelation of their mutual identities has come too late, after they have fallen irrevocably in love with each other. Thus after the Nurse, in her playful and gossipy dialogue with Romeo, reveals Juliet's identity, he responds: "O dear account! my life is my foe's debt". There is also dramatic irony in Juliet's dialogue, as Juliet says as she sends off Nurse to find out Romeo's identity "If he be married / my grave is like to be my wedding bed". We of course know that Romeo is not married, but Juliet is startlingly accurate with the latter part of her sentence. Juliet responds to Romeo's identity as follows:

My only love sprung from my only hate!
Too early seen unknown, and known too late!
Prodigious birth of love it is to me,
That I must love a loathed enemy.

Thus Shakespeare makes it clear that it is too late - the lovers cannot forget their "birth of love" and we are drawn in to see what will happen next between these two "star crossed lovers".