Butler paid tribute on Instagram, writing alongside a video that Nyqvist was “incredibly talented and an extraordinary human being. My heart is broken.”
The Oscar-winning actress said that adults today are “a little lost” and that children are needed to help in the future.
The creator of the world’s most famous boy wizard marked the anniversary of her characters being shared with others.
The Luna Lovegood actress was once a pen pal of author JK Rowling before she was cast in the film versions of the books.