I'm not sure that I understand your question. Are you confusing actor and character? Actors play characters created by a playwright, in this case William Shakespeare.
The job of the actor is to take all the information about his/her character and with the help of the director bring that character to life on stage for an audience.
What does the actor look for? There are several ways an actor discovers character. What does the playwright tell you about the character directly, IE, "Enter Juliet a teenage girl, etc." Shakespeare gives no specific information of this nature. More important information is contained in what the character actually says and does. Actions speak louder than words. Does the character do what he says he will do? Another clue is what other characters say about your character and their relationship. For example, Tybalt has nothing good to say about Romeo, so that information is not very valid.
Your movie question is another question and in this case you are not very specific as to which film. There is more than one.
Juliet is a naive 13 year old girl, who has gone completely bonkers because some really cute guy likes her. She is smart, yet not smart enough to realize the trouble she's getting in for being with Romeo. She is also very strong (and strong hearted, because she chooses a violent death, as opposed to Romeo, who merely takes the poison).
Romeo is a confused and hormonal sixteen year old boy who was CONVINCED the world would end because he couldn't be with his love, Rosaline. However, laying eyes upon Juliet's pretty face changes his mind for him! All he really wants to do though, is get in her pants (there is a part, later in the play, where he, benvolio, and mercutio are talking about "the pink flower" and so on, which is basically talking about Juliet's virginity, and how he took it.) He is a P-A-N-S-Y. He is always crying, and expressing his emotions extremely, and he can't even take a dagger to the heart like Juliet can!