i'd say all or nothing. you have to be willing to risk what you may already have or don't have. if you are willing to risk it, then i would tell you to straight up tell her that you like her and if she would want to go to the movies or out to lunch. start small then work you way up.
about her acting different infront of your friends and what not, i think she is just a shy person that it will take some getting use to. she might not have had a lot of relationships or maybe she's just shy in general. either way it will get better with time. raverboy
...this is just my perspective on the situation...