Judy Blume recalls demonstrating iconic 'increase our bust' scene on-set for her new movie

Celebrated author Judy Blume has sold over 90 million copies of books to generations of readers worldwide, and soon moviegoers will get a glimpse of one of her classic coming of age stories, "Are you there God? It's me Margaret," on the big screen.

The author said she lit up about the idea of the film adaptation for the first time when Kelly Fremon Craig, the writer and director of "Edge of Seventeen," sent a "wonderful letter" to her about it.

"This is the first time I have ever had someone ask me, 'Can we make the movie?' with that kind of credential," Blume told "Good Morning America." "And to top it off, she said, 'My mentor is [Academy Award-winning director] James L. Brooks and he is with me the whole time ... can we come to meet you in Key West?' ... By the end of that lunch, we all knew we were gonna do it."

"I may be one of the only authors who says, 'this movie is better than the book,'" Blume added.

"Are you there God? It's me Margaret" was written over 50 years ago and includes an iconic line that Blume, who joined as the movie as a producer, was emphatic they get right for the film: "We must, we must, we must increase our bust."

"I was sitting next to one of the real producers, Julianne. I was there on the set watching it on the monitor and suddenly the girls started, 'I must, I must, I must increase my bust.' I'm like, 'Wait, stop! You're doing it the wrong way,'" she recalled telling the actors. "That's the way Kelly did it when she was growing up. Nobody told her the right way. So, everything stopped and I gave it a little demonstration."

Blume said the story's appeal and ability to connect with young readers across generations is in part thanks to a sort of nostalgia.

"Some people just have a great memory for their own childhoods, and I had that. When I started to write, it never occurred to me to write anything except about kids. Kids on the cusp, kids on the edge. And I think that's part of the connection," she explained.

In some places across the U.S. however, Blume's books have been banned from school reading lists.

"I went through the '80s and we thought the '80s ... might be as bad as it would get. We're America, right? We're supposed to have intellectual freedom," she said. "And now it's back and it's worse. It's worse because it's coming from the government and it's coming from elected legislators."

Blume, who lives in Florida, a state where books like hers have been banned in schools altogether by Republican legislators who claim literature that deals with issues such as sex, gender or race is inappropriate for children, added, "It's terrible and we have to fight it -- speak out, yell."

A new documentary about Blume's life, "Judy Blume Forever," is currently streaming on Prime Video. Blume said it feels strange to talk about "because that's me on the screen."

The author said she appreciates that the documentary includes "letters from the kids over so many years, thousands and thousands of letters, and they show you some of the kids who wrote to me who were kids [then], who are now grown-ups."

Blume said she still receives letters, but more are sent via email these days.

"Are You There God? It's Me, Margaret" premieres in theaters Friday, April 28.