HBO's Harry Potter series has confirmed the casting of new characters, with some outlets reporting that 21 new cast members have been added. The new cast members are said to include students from Hogwarts houses Gryffindor, Hufflepuff, Ravenclaw, and Slytherin. However, there is some uncertainty about the exact number of cast members confirmed, with one outlet reporting that only the first six cast members have been announced, including key characters like Hagrid and Snape.