Dramatic irony can be found in a drama when the characters on the stage know less about what has already unfolded, or is currently unfolding, in the plot than the audience. Dramatic irony can be identified in the words the characters are speaking, or in the situation being presented as a whole.

In this scene, there is a lot the characters onstage don't know which we, as the audience, do. Firstly, the situation itself is an example of dramatic irony in that we as the audience know that Juliet is not dead at all, but has taken a special potion given to her by Friar Laurence. This means that the mourning into which the Nurse and Lady Capulet immediately fly over Juliet's apparent death is ironic. The double irony, however, lies in the fact that Capulet's comments may not be apposite here, exactly, but when he comments that "death lies on her like an untimely frost," this will certainly be true within a few scenes. So there is prophetic irony in the statements too.

Later, there is also a further element of irony to be found in the discussion Capulet has with Paris about their "wedding day." Capulet says that Death has "lain with [Paris's] wife." As members of the audience, we know that Death has not actually got his figurative hands on Juliet yet—but Romeo has. Paris was still expecting to be able to marry Juliet today, but actually she is already married to Romeo and thus could never have been his wife anyway.

A playwright creates dramatic irony when there is some bit of knowledge that the audience is given but that is unknown to one or more of the characters. This can be used to many effects: it can be funny, as when Falstaff hides from Ford in a laundry basket in “The Merry Wives of Windsor” – we know Falstaff is there, but Ford doesn’t and allows Falstaff to escape. It’s also used in nearly every horror film, when the hero is talking on the phone, for instance, and doesn’t notice the monster behind him, but we do and the monster is getting closer…

In Act 4, scene 5 of “Romeo and Juliet” we see Friar Laurence’s plan put into action and come off brilliantly: Juliet drinks the potion and appears to be dead, and her entire family believes her to be dead indeed. We don’t share in the family’s grief, because we know all about the plan and we watched Juliet take the potion and know she’s going to be fine. For the family, this is where tragedy strikes but for us this is all part of the plan – we don’t experience Juliet’s tragic end. Not yet.

Firstly, the irony lies in the fact that Lord and Lady Capulet both believe that their daughter has died whilst we, the audience, know that she is only in a state of unconsciousness - a sleep so deep that she seems dead. She had imbibed a potion given to her by Friar Laurence as part of their plan for her to be with Romeo, who was to fetch her later from the family burial-vault after she had been interred.

Furthermore, Lord Capulet's assertion that death has now become his son-in-law adds to the irony, for the reason expressed above.

Further irony lies in the fact that Lord Capulet, addressing Paris says:

"... the day before thy wedding day

 Hath Death lain with thy wife."

The audience knows that Romeo and Juliet had already been married in Friar Laurence's cell and Juliet could therefore not be Paris' wife. Nor could she become his, since she is already betrothed. 

More irony lies in the fact that Lord and Lady Capulet's grief at this particular moment is not warranted, but they will experience even greater (and truer) grief at the end when they discover the true tragedy of their daughter's demise: that she had killed herself because of Romeo's death, but more, that she was lead to these actions because of the strife between the houses Montague and Capulet, something for which they were responsible. In this sense then, they were indirectly responsible for her death - in that lies the greatest irony.

Act 4, scene 5 is the act in which Juliet is found to be "dead" by the nurse. Dramatic irony exists in a play when the audience knows something about a situation that the characters in the play do not have knowledge about. In this situation, the audience knows that Juliet's death is not a real suicide at this point, but that she has taken a potion given to her by Friar Lawrence to make her look and seem as though she is sick for forty-two hours, until Friar Lawrence has enough time to remedy the other situation that is taking place in the play. When the nurse finds Juliet, neither she nor Juliet's parents know that Juliet is not really dead -- making this scene an example of dramatic irony.