De esta entrevista de JKR (obrigado, Dason):
In the fifth book, Harry can see the Thestrals. Can you?
Yes, I can, definitely. That is a really good question, because it enables me to clear up a point. The letters that I’ve had about the Thestrals! Everyone has said to me that Harry saw people die before could see the Thestrals. Just to clear this up once and for all, this was not a mistake. I would be the first to say that I have made mistakes in the books, but this was not a mistake. I really thought this one through. Harry did not see his parents die. He was one year old and in a cot at the time. Although you never see that scene, I wrote it and then cut it. He didn’t see it; he was too young to appreciate it. When you find out about the Thestrals, you find that you can see them only when you really understand death in a broader sense, when you really know what it means. Someone said that Harry saw Quirrell die, but that is not true. He was unconscious when Quirrell died, in Philosopher’s Stone. He did not know until he came around that Quirrell had died when Voldemort left his body. Then you have Cedric. With Cedric, fair point. Harry had just seen Cedric die when he got back into the carriages to go back to Hogsmeade station. I thought about that at the end of Goblet, because I have known from the word go what was drawing the carriages. From Chamber of Secrets, in which there are carriages drawn by invisible things, I have known what was there. I decided that it would be an odd thing to do right at the end of a book. Anyone who has suffered a bereavement knows that there is the immediate shock but that it takes a little while to appreciate fully that you will never see that person again. Until that had happened, I did not think that Harry could see the Thestrals. That means that when he goes back, he saw these spooky things. It set the tone for Phoenix, which is a much darker book.
Eu acredito que ver alguém passar pelo véu poderia permitir que a pessoa visse os thestrals, se eles "entendessem a morte em um sentido mais amplo" e / ou "apreciassem que eles nunca verão essa pessoa novamente".