"When you're cutting an entire novel into a two-hour film, there have to be edits and sometimes the author needs to be open to adaptation," he said.
But JK has been given much more control than usual, but he is relaxed about that. He has never thought about messing around with JK Rowling's storylines in making the films.
So far, the Potter producer has not had to worry too much about millions of Potter fans trying to find the bits of the films that don't match the books.
Harry Potter and the Philosopher's Stone and Harry Potter and the Chamber of Secrets has so far made £982 million ($1.8 billion) at cinema box offices.
Heyman is currently finishing off Azkaban, and at the same time has started working on pre-production - the bits that start before actual filming - on the fourth film, Harry Potter and the Goblet of Fire.