They have voice coaches. I think it is pretty amazing. Hugh Laurie as Dr House sounds very American. Listen to him as a guest on a talk show and his voice is so different sounding. Accents are interesting. In Northern New York if you pass over the US-Canadian Border on land into Quebec or over a bridge on the St Lawrence River into Ontario the accent is so obvious yet we live so close to each other and watch the same TV shows and listen to the same radio stations.