Friar Lawrence believes Romeo has sinned because Romeo hastily switched his affections from Rosaline to Juliet, marrying her impulsively. This goes against conventional morality and the Friar is concerned about the consequences of such reckless behavior.
At various times, Benvolio, Mercutio, the Nurse, Friar Lawrence, Romeo's father, Friar Lawrence's friend Friar John and Romeo's servant Balthazar all try to do things which they think will help Romeo. And Juliet too, of course.
he thought he had spent the night with Rosaline
Romeo believes that Friar Lawrence cannot understand his point of view because the friar is not in love with Juliet like Romeo is. Romeo feels that his intense emotions and passion make his situation unique and beyond the friar's comprehension.
Friar Lawrence believes that Romeo has been with Rosaline all night, as he is unaware of Romeo's newfound love for Juliet. He is surprised to see Romeo so lovesick and assumes he is still pining for Rosaline, demonstrating the Friar's lack of knowledge about the rapid shift in Romeo's affections. This misunderstanding highlights the theme of love's impulsiveness in the play.
Romeo and Juliet, obviously. Lady Capulet, Lord Capulet, the nurse, Mercutio, Tybalt, Friar Lawrence and Benvolio. I think that's all
Romeo was in Mantua at the time, because he was had to flee Verona after he was banished for fighting and killing Tybalt it the street. Friar Laurence wasn't the one who was sent to find Romeo though, Friar John was, but he was detained and the letter never reached Romeo.
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.
When we first meet the friar, Romeo is coming to tell him about his love for Julilet and how he wants to marry her. The last the friar heard, Romeo was in love with Rosaline.
Friar Laurence at first believes Romeo is still pining over Rosaline, a woman who does not return Romeo's love because she says she has taken a vow of chastity (some think this means she has decided to become a nun). In fact, Romeo has completely forgotten about Rosaline and is now in love with Juliet, but since Juliet is the daughter of his father's enemy (and Rosaline was only his neice), this poses a major problem for Romeo.
Friar Lawrence is a priest, and priests are always called "Father". To understand why "ghostly", you need to know something about the word "ghost". It means the same as "spirit" or "soul". That's why we talk about someone "giving up the ghost" when they die; they are letting go of their soul. That's why the "Holy Spirit" in Christian theology is sometimes called the "Holy Ghost". Ghosts in the sense we usually think of them are the spirits of the dead who cannot rest, which is why the Ghost in Hamlet says "I am thy father's spirit". Friar Lawrence is not Romeo's real father--that is Montague. But the Friar is Romeo's spiritual father, the one Romeo goes to for advice and support. Spiritual and ghostly, as we have seen, mean the same thing. Hence, "ghostly father."
Friar Lawrence gave Juliet a potion that would put Juliet into a coma-like sleep for about two days straight. Everybody would think she was dead. Juliet's relatives would put her in a cellar (their family morgue) and Romeo and himself would be there waiting for her to wake up. Then Romeo and Juliet could live together in Mantua.
In ACT 2 Scene 3 of Romeo and Juliet, Romeo goes to see Friar Laurence because he wants to marry Juliet. In Act 2 scene 2 Romeo exchanged vows of their love. So Juliet told Romeo to tell her where and when they are to be married. She would send a servant to Romeo to find out. Romeo wants to make sure that Friar Laurence can marry them. Friar Laurence said he would marry them, that very day. Hope that helped answer your question!