JK Rowling has revealed how difficult it was to adapt her novel Fantastic Beasts And Where To Find Them into the hotly anticipated new film.

The Harry Potter author said she managed in part by treating the script like a book, including not just dialogue but long descriptive passages about the setting and characters.

Speaking on stage at Carnegie Hall in New York ahead of an advance screening of the film for her foundation Lumos, JK said: “It’s like learning a completely new language. I learned to write a screenplay while writing a screenplay.”

The film was directed by David Yates and stars Eddie Redmayne, Alison Sudol, Dan Fogler, Katherine Waterston and Ezra Miller.

The author started Lumos a decade ago to help institutionalised children worldwide be reunited with their families.

Her voice was hoarse from days of promoting the new film and JK joked that she was “full of honey” as she joined Eddie for a conversation about her charitable work and her script.

She has often related her inspiration for Lumos, saying she was reading the newspaper and spotted, to her horror, a picture of a child in a cage.

Unsure if she could bear to keep reading, she told herself that if the story was as awful as the picture suggested, she had no choice but to do something about it.

“They are so voiceless,” she said of the children in orphanages, which she has criticised often as damaging to children and their development.

“This is an extremely solvable issue,” she said. “It doesn’t mean it’s easy, but we know how to do it.”

Fantastic Beasts And Where To Find Them is released in the UK on November 18.