Sometimes you meet someone, and they seem great, they seem exactly what you're thinking of for the role, and then you put a camera on them, and they freeze. And other people come to life with the camera on them. I haven't discovered any reliable predictor for that; I think you just have to try it and see what happens. And, you know, sometimes the people who freeze, if you find the right magic word to say to them, you can unlock them.