How does Shakespeare make it interesting and tense for the audience at the end of Act 1 scene 5, when the Nurse reveals their identities?
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. Of course, one of the central methods Shakespeare uses to make the scene tense is dramatic irony: we as the audience know the identity of the two lovers, when they do not, and we know that they are from feuding households.
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:
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".