"Harry Potter and the Order of the Phoenix," the fifth installment in Rowling's wildly popular children's series about a boy wizard, comes out Saturday.
Its plot is a closely guarded secret, but Rowling has said one of the central characters dies in the book. She hasn't even told her husband, Neil Murray, which one it is, she said in a British Broadcasting Corp. interview airing Thursday. The BBC released excerpts in advance.
After writing the death scene, Rowling recalled, "I walked into the kitchen crying and Neil said to me, `What on earth is wrong?' And I said, `I've just killed the person."'
"And he said, `Well, don't do it then,"' she continued. "And I said, 'Well, it just doesn't work like that.' You are writing children's books, you need to be a ruthless killer."
She said there's a crucial Potter secret yet to be revealed, but gave no hints for eager readers.
"There is one thing that, if anyone guessed, I would be really annoyed, as it is kind of the heart of it all," the author said. "And it kind of explodes everything. And no one's quite got there, but a couple of people have skirted it."
Rowling vehemently rebutted rumors that she suffered from writer's block while creating the hefty volume.
"Just once and for all, for the record, I didn't miss the deadline. Because there was no deadline," she said. "I just produced a quarter of a million words!"
Rowling plans to write seven books total about the adventures of Harry and his friends at the Hogwarts School of Witchcraft and Wizardry. She told the BBC she has mixed feelings about eventually finishing.
"I do look forward to a post-Harry era in my life, because some of the things that go along with this are not as much fun," she said. "But at the same time, I dread leaving Harry. ... I worked on it so hard for so long ... then it will be over and I think it's going to leave a massive gap."