Here's the one thing that stumped 'Harry Potter' filmmakers

J.K. Rowling mostly kept her hands off the "Harry Potter" movies. She already said everything in the books.

She played close attention to certain details, and the filmmakers sought her approval, but for the most part, she left the people in charge of translating her magical world to film alone. They had thousands of pages of source material to work from.

"She was really happy to let everyone interpret them in their own way, and for each department," Miraphora Mina told INSIDER. Mina co-founded MinaLima, the graphic design firm that designed everything in the "Harry Potter" movies from Harry's Hogwarts' envelope to Voldemort's Horcruxes.

There was one part where Mina needed J.K. Rowling's help: The Black family tree.

The tree is in 12 Grimmauld Place, a magically disguised hideout in London where Harry first stays at the beginning of "Harry Potter and the Order of the Phoenix." It's also the ancestral home of the Black family, of which Sirius Black, Harry's godfather, was a member. Most of the family was obsessed with their pure-blooded status (meaning they didn't have many muggles in the family), and they had an elaborate family tree on a tapestry to celebrate it. Sirius was erased from the tree when he ran away from home as a child.

"Harry Potter and the Order of the Phoenix" doesn't say too much about what's on the tree itself, although it does describe the tapestry:

The tapestry looked immensely old; it was faded and looked as though doxies had gnawed it in places; nevertheless, the golden thread with which it was embroidered still glinted brightly enough to show them a sprawling family tree dating back (as far as Harry could tell) to the Middle Ages.

Instead of appearing as a tapestry taking up a single wall, it appears in the movie as a wallpaper occupying an entire room. But Mina and her work partner, Eduardo Lima, didn't know what to put on it. Neither the book or script had the details.

"It says 'there is a family tree,'" Mina said. "But our job as graphic designers is to present the whole thing, and we didn't know who was related to who."

So they asked Rowling for help, and she provided an elaborate diagram of everyone's relationship with each other.

And just like that, new information entered the "Harry Potter" canon. It not only accurately included the minute details of the tree from the books (like of Alphard Black being blacked out for taking Sirius in), it also added several generations of previously unknown Black family members.

It isn't the only time Rowling weighed in on designs in the "Harry Potter" movies, but in general, Mina said she was "generous and approving" of her own designs.

"I think she loved it all. It must have been strange for her to see it realized in material form," she said. "She even took a couple of books away, that we've made ... It's the ultimate compliment."

In the forthcoming "Fantastic Beasts and Where to Find Them" movies, Rowling will have more involvement than she did in the "Harry Potter" ones. For one, Rowling wrote the script herself this time. She'll have control over every detail of the magical universe from the start.