harry potter - Just how much of The Cursed Child is JKR's?

Harry Potter and the Cursed Child is marketed as

The eighth story. Nineteen years later.

In big block caps on the back of the book.

The front cover also proclaims

Based on an original new story by

J.K. Rowling

John Tiffany & Jack Thorne

A new play by Jack Thorne

With Rowling's name being very much more prominent.

But it's very clear that Rowling didn't actually write the play.

So, do we have anything more concrete than "based on an original new story", do we know how much she had to do with the plot and the development of the characters, and the big reveals like

Delphi being Voldemort's daughter.

Answer

It would appear that JKR co-authored the play's story (which presumably includes character creation) but Jack Thorne wrote the actual play. Consultation was held at each stage of the play's development, with Jack Thorne was responsible for the actual dialogue with John Tiffany responsible for the staging. JKR would provide extensive notes but doesn't seem to have done rewrites.

You both share story credit with J. K. Rowling. How did it work having three writers in the mix?

John Tiffany: Jo Rowling was incredibly generous. I met her first, and I already had a soft spot for her because she used to write in the cafe of the Traverse Theater in Edinburgh when I was the director. It was only after the first book came out that I realized it had been her, nursing one cappuccino for four hours. When we met to talk about the play, she asked, “What do you think the Harry Potter stories are about?” I said, “Learning to deal with death and grief.” There was something in her eye — I thought, we didn’t say it’s about transformation or magic or flying on brooms, and we’re on the right track.

Thorne: We all met in Edinburgh and as the day developed, we knew we would take the epilogue of “Harry Potter and the Deathly Hallows” as a starting point.

Tiffany: All the seeds are there; we start with that scene in the train station. Am I allowed to say that? Anyway, it was clear that she was going to let us take those characters and have our own ideas.

Callender: Of course, Jack came to the table with an encyclopedic knowledge of Harry, so that helped.

Thorne: All right, I’m a nerd. With abandonment issues.

Q. Did you really sketch out the whole arc of the story in that first meeting?

Thorne: Yes, but then it took about six months to really map the whole thing out. Every time it was like taking a big step forward, one or two small ones back. Jo would say, “This feels right, this doesn’t.”

Tiffany: There are parts of the story, which when we first conceived them, I didn’t think she would let us do, but she never hesitated. It is one thing to let us continue the story, another to let us unravel the canon.

_{^{Why J. K. Rowling Endorsed ‘Harry Potter and the Cursed Child’ for the Stage}}

and

John Tiffany explains: "We didn’t start writing the play – or Jack didn’t – until we’d agreed on what that story was." Jack Thorne then started on writing the script that is soon to be published in (script)book form.

_{^{“I didn’t want to let fans down”: J.K. Rowling, John Tiffany and Jack Thorne on Creating ‘Cursed Child’}}

and

The three of them set the plot that day in J.K. Rowling’s writing room. They strung together the narrative then and there in notebooks and then Jack and John flew back to London to get started.

‘We talked a lot, which was really helpful,’ says Jack, when I ask him about the task of writing dialogue for Harry Potter. ‘I read the books over and over and then tried to write things down on paper...’ He sighs. ‘Ah, it’s awesome, I mean, it’s so much fun.’

_{^{Pottermore: Cursed Child creatives on collaborating with J.K. Rowling}}