There are some good cast choices. First I have no idea exactly how the Ainur would be portrayed on film. Tolkien describes Sauron as being able to take on guises as beautiful until he wsa caught in the fall of Numenor. As for Melkor, I like Fassbender. We have to keep in mind that Melkor was THE most powerful of the Ainur to come to Middle Earth and, with Manwe, may have been the strongest spirit created by Illuvatar in the beginning of all things. Keep in mind that Middle Earth was created by the great Song of the Ainur wherein each of the Aunir knew only a part of Eru's mind and each sang his/her part. However, Melkor was proud and tried to change the themes to the way he liked them. So there was strife in the making. Melkor was ashamed and hid. When Eru made the song into the physical Earth (Middle Earth) many of the Ainur desired to go down to it and order it according to the vision they were shown by Illuvatar. Melkor and the Ainur he convinced to follow him in the song also went. So we have a very powerful being who is proud, and in the beginning of his existence, childish. Arrogant yet that sometimes backfires on him as in the case of Ungoliant. I don't think Melkor starts out completely evil, but in his effort to get even for his shame he does great evil until, like many evil men, he fully believes that what he does is righteous. Tolkien, in the book Morgoth's Ring, makes the case that Melkor put so much of his power into marring the things Eru brought to Middle Earth, that Middle Earth became for Melkor Sauron's ring. I really think Fassbender can play this childish malevolence that Melkor had in the beginning, like a mean kid burning things with a magnifying glass. There are other actors who can pull this off, but I think Michael would be magnificent in this role.