'Are You There God? It's Me, Margaret' Director on 'Pressure' of Honoring Judy Blume's Book (Exclusive)

The groundbreaking book became a favorite for millions of girls for its frank depiction of first periods and shopping for bras

Director Kelly Fremon Craig still remembers a conversation she had at 11 years old, when a friend came to her and said, "You've got to read this book. It's this group of girls, and they talk about periods!"

The book, of course, was Are You There God? It's Me, Margaret, the classic 1970 novel by Judy Blume about a girl on the verge of puberty dealing with both universal coming-of-age themes (family, friends, identity) and the stuff nobody had talked about before in fiction for children: periods, bras and the silent late-night conversations with God about desperately wanting to fit in.

After that, Craig tells PEOPLE, "I just was on a Judy Blume kick and read everything."

Still, Margaret was special: "I think because at the time, when I was that age, I was just so awkward."

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Related: See Rachel McAdams and Kathy Bates in Are You There God? It's Me, Margaret First Look

So when Craig, fresh off the success of her directorial debut The Edge of Seventeen, began to ponder her next project, it was natural that she'd turn to her childhood favorite.

She and her mentor, the legendary director and producer James L. Brooks (The Mary Tyler Moore Show, Terms of Endearment, Big, Jerry Maguire), sent Blume, 85, an email expressing their interest in bringing Margaret to the big screen.

It was not a sure thing that she'd say yes. After all, the book had been out for half a century, and nobody had filmed it yet. But it helped that Blume had seen The Edge of Seventeen even before getting the email from Craig and Brooks.

"I had vivid memories of scenes in that movie," Blume tells PEOPLE. "And I thought, 'This is really the first time that anyone has come to me with those credentials.' She's written and directed a feature that I saw and I loved. Then she said to me, 'And James L. Brooks is my mentor.' "

"I was afraid to let it go for 50 years," the author adds — but it involved the trusted the team of Craig and Brooks, and so she said yes.

Related: Are You There God? It's Me, Margaret Trailer Show Iconic Judy Blume Book Brought to Life

And then the pressure began — not only to get things right for the book and its millions of devoted fans, but also to make Blume feel she'd made the right choice in letting them make the movie at all.

"You don't want to be the person who does wrong by Judy," says Brooks. "So I felt a pressure, I felt an anxiety about that throughout."

"I felt it as absolutely a crushing pressure to begin with," says Craig. "But I shed it at some point, because you can't operate like that."

She adds, "But the first few weeks of writing the script, the pressure was paralyzing."

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Even Abby Ryder Fortson, who plays Margaret in the film, felt it. "She's a legend and one of the kindest people that you will ever meet," she tells PEOPLE of Blume.

After she read Are You There God? It's Me, Margaret, Fortson felt it even more. "My mom had read the book when she was young. I had never read it," she says. "And when I got the audition for it, I immediately read the book. And then my reaction was like, 'Oh my God, someone wrote this down. How is this possible?' "

And once she had been grabbed by the book, Fortson, too, worried about whether the film would please its author.

"I think we all were like, 'Oh my God, is she liking this?' " she says. "We all want to do right by her because it was her baby. Margaret was her baby. She really just nurtured Margaret, and it was her book, and we all just really wanted to do it justice."

Blume spent five weeks on set with the cast and crew, watching the process as the film was made. "It was so fascinating to me," Blume says. "It takes so much to make a good movie. You wonder how it ever happens."

She was impressed with how well Brooks and Craig created an atmosphere for trust for everyone involved in the film. "Kelly's set was so calm," Blume says. "Especially [for] the young actors, I think they felt safe, which was so important."

In the end, the time together made them "a family," Blume says.

Working together in a collaborative and warm process, having the author on-set, did its job. "I just think that it was innate guidance," Brooks says. "Something about her was a lighthouse for us."

Adds Fortson, "We were also happy when she put her stamp of approval on it."

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

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