Historically speaking, animating female characters are really, really difficult, 'cause they have to go through these range of emotions, but they're very, very — you have to keep them pretty and they're very sensitive to — you can get them off a model very quickly. So, having a film with two hero female characters was really tough, and having them both in the scene and look very different if they're echoing the same expression; that Elsa looking angry looks different from Anna (Kristen Bell) being angry.