One good example of dialogue showing appearance and reality comes in act 4, scene 1, when Paris runs into Juliet when she goes to meet with Friar Laurence. Paris is smitten with her and wants to chat, while Juliet, despairing over Romeo's banishment, just wants to meet with the friar already. When she tells Paris she is there to confess to the friar, Paris flirts and says, "Do not deny to him that you love me," to which Juliet replies, "I will confess to you that I love him."
Paris believes she is flirting back, but the audience knows Juliet is talking about Romeo, the man she really loves. (This echoes the earlier scene with Juliet's mother, mentioned in one of the previous answers. Juliet uses misleading wording, technically telling the truth about her feelings but hiding them from her family.)
An earlier and more famous example would be during the balcony scene in act 2, scene 1, when Juliet declares, "A rose by any other name would smell as sweet." In effect, she is saying that Romeo's familial affiliation does not mask his good nature to her. Even though the name Montague appears hateful, in truth, Romeo is not a bad person.