Fans from Hong Kong dress up like the boy wizard on the day of the release of the published version of the Harry Potter and the Cursed Child stage play. [Photo/Agencies]

Co-producer Sonia Friedman said she had big hopes for the show, which has been more than two years in the making.

"Hopefully many countries at some point will get to see it," Friedman said. "But it's a big piece of theater, it's a big endeavor. You can't just turn this around overnight."

The play has been in previews at the theater for almost eight weeks, but few details of the plot have leaked (though those seeking spoilers online will find them).

People leaving the show are handed buttons urging them to #keepthesecrets－and most have complied.

Tiffany said spilling the play's secrets was "like unwrapping children's Christmas presents for them in November, in front of their eyes. And why would anyone want to do that?"

Rowling said fans have "been amazing, they've been incredible."

"It is the most extraordinary fandom, so I'm kind of not surprised, because they didn't want to spoil it for each other," she said. "But I am so happy we got here without ruining everything."