Cardcaptor Sakura

I love to watch Japanese animation. The first two I ever got into was Yakitate Japan and Akagi. One is about a baking bread competition and the other, mahjong competition. I guess competition is my thing. Since then I've watched many anime shows from all sorts of genres, hundreds of episodes, but something has started niggling in the back of my mind. When I see a character that is Japanese but has green eyes and blonde hair, it makes me think. Why is that character portrayed like that?

I ask this because the problem seems to come to a head when the anime is being adapted by a studio. An American studio. Then who's going to be cast comes into play. Do the cast directors ask for and choose the actors who look like the characters? Do they choose Japanese actors and tell them to dye their hair and wear contacts? Do they choose Japanese actors and let the character look however the actor looks?