When I cosplayed as Mulan, it was not that hard for me to stay in character. I stayed true to the movie, which I have seen a dozen times (I can even quote most of it) and as I am very similar to Mulan myself, it wasn't that hard for me.
I think in our group, most of us created their Disney princess after their own imagination of the role. Our Cinderella, for example tried to always stay calm and gentle (which was very hard, she often complained that she couldn't curse xD). But we also allowed ourselves some OOC-moments when we just fooled around, like for example discussing who has the best prince and stuff