What is Juliet like when we meet her in Act 1 in Romeo and Juliet, and how has she changed by the end of Act 1?

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:

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...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.