Thank you for posting on Wyzant. I would like to give you advice on this subject.
When looking for the main characters of a scene in a play, book, or movie you should focus on what character(s) is presented the most. It can be in both action and in words.
Example: There are three characters in a book. Which one would be considered the main one?
"It's quiet a sunny day," Jack said turning to Josh who was looking up at the bright sky and thinking about what kind of day it would turn out to be.
"I don't quite think so," from the side replied Emmy. Josh considered her to be quiet a negative person, but Jack seemed to appreciate her all the same.
"Well, Josh seems to think so," (Jack wrapped his hand around Josh's shoulders and squeezed it.) "Don't you buddy?" Josh was only thinking about how uncomfortable this position was.
Answer: Josh is the main character, because it refers to his thoughts more. (Note: this can also be considered with actions.)
Referring to Romeo and Juliet, in the beginning of Scene 1 consider what two characters are present and more demanding, have more action or thought, or appear to appear in the scene most often.
If you need more help, I am happy to have a lesson with you and teach you the skills to help improve your writing, both an style and speed. I also just recently read Romeo and Juliet with one of my students.
Please let me know if you are interested or need some more help with questions.