Sorry, only one question per submission, so I took your first question about Juliet in Act I. For more information on Juliet's relationships with her parents, please re-submit separate questions.
Juliet first appears in Act I, scene iii, but, if number of lines spoken indicates the central characters of a scene, she isn't really the focus of the action here. Lady Capulet and The Nurse do most of the talking in this scene. However, Juliet's silence is actually telling. It would have been proper, during the time that Shakespeare lived and worked, for a young woman Juliet's age to remain quiet and answer questions only when addressed. She appears to be just such a proper young woman in the beginning of the scene. When called, she dutifully asks her mother, "What is your will?"
The next time she speaks is to ask the Nurse to be quiet and then to respond to her mother's question about her interest in being married. She says, "It is an honor that I dream not of," indicating that she hasn't been thinking about marriage at all. So, we seem to have here a dutiful young woman, one not yet ready for the serious step of marriage.
The lone spark of the headstrong and decisive girl that Juliet shows herself to be later in the play comes at the end of the scene, when Juliet responds to her mother's question about loving Paris, a man she's never met with the line:
I'll look to like, if looking liking move,
But no more deep will I endart mine eye
Than your consent gives strength to make it fly.
This indicates that she either doesn't trust her mother's opinion or simply intends to disregard it, an opinion and comment one wouldn't expect from the dutiful young girl she has seemed to be up to this point.
It is in Act I, scene v that she meets Romeo and her disinterest in marriage is quickly overturned. She exchanges words of attraction with Romeo in lines 95 - 109, and, at the end of the scene confesses the depth of her interest in Romeo when, as she sends the Nurse away to learn his name, she says:
...If he be married,
My grave is like to be my wedding bed.
Once she learns that he is a Montague, the die is cast and she confesses her love:
My only love sprung from my only hate.
Too early seen unknown, and known too late.
Prodigious birth of love it is to me
That I must love a loathed enemy.
So, from her first entrance in Act I, scene iii as a girl who had no thoughts of marriage to the smitten young woman of Act I, scene v, Juliet undergoes an awakening -- she is in love.