The friar and Juliet can control their actions and decisions within the plan, such as Juliet taking the potion and pretending to be dead. They can also control how they communicate with each other to ensure the plan is executed smoothly. However, they cannot control external factors or other people's actions that may impact the outcome of the plan.
Balthasar ruins the friar's plan by beating Friar John to delivering the message of Juliet's fake death. By telling Romeo before the Friar it caused Romeo to believe Juliet was actually dead.
Romeo wants the Friar to marry him to Juliet.
Friar Lawerence's plan was for Romeo to take Juliet away to Mantua with him where they could live together.
Friar Laurence asks Friar John to deliver a letter to Romeo explaining the plan to fake Juliet's death.
Juliet threat to kill her self
Juliet and Friar Lawrence made the plan to fake Juliet's death so she could runaway with Romeo. They had planed for her to take a vile that would make her cold and fall asleep for 48 hours. Friar Lawrence sent a note to Romeo telling of their plan so Romeo and Friar Lawrence could go retrieve Juliet once her funeral was over.
She was anxious and desperate. If the Friar had not thought of a plan her intent was to kill herself, right there.
Juliet's emergency with Paris.
She threatens to kill herself.
His plan is to hide in the tomb with Juliet. That should have been his plan all along, and he should have got on it sooner.
In scene 2 of Romeo and Juliet, the unseen occurrence that ruins Friar Laurence's plan is Friar John being unable to deliver the letter to Romeo in time. This prevents Romeo from receiving the message from Friar Laurence explaining Juliet's fake death plan, ultimately leading to tragic consequences.
The Friar knows that Juliet is already married. He doesn't want to have to refuse to marry her to Paris in a public way. His plan not only helps Juliet, it also gets him off the hook.