Look at the insults Juliet throws at Romeo in A3S2. Write to either Romeo or Juliet, offering him/her advice about what they should have done.
Act 3 Scene 2 of the play focuses on Juliet's response to the news of violence from Act 3 Scene 1. In Scene 1, Romeo witnesses Tybalt's murder of Mercutio, and in return Romeo avenges the death with the Murder of Tybalt. During Scene 1, Romeo hurls some of his own insults towards Juliet, claiming their love has made him effeminate and without courage.
In Scene 2, Juliet is impatiently waiting to see her new husband Romeo that night, but instead the Nurse appears claiming, "He's dead, He's Dead!" Considering her focus on Romeo, Juliet assumes the Nurse is speaking of Romeo's death. Juliet then has to fight her mixed emotions as she feels relief that Romeo is alive, grief and anger that her cousin Tybalt is dead, and then regret for her negative feelings towards Romeo. It is quite a bit to take in for such a short scene.
When Juliet first learns Romeo is responsible for Tybalt's death, she uses the following paradoxes to describe her mixed emotions. I have added comments to help, but she continuously repeats the idea of beauty hiding an evil in Romeo:
In your letter, I would focus on giving Romeo or Juliet advice on staying level. They show great emotional immaturity, and they allow themselves to act and react based on emotion without thinking things through. Perhaps writing to Juliet and asking her if she feels she really knows Romeo or is she simply blinded by his beauty and attracted to him. Or you could write to Romeo and ask if this new found love is worth the price of his best friend and murder.