Lady Diana Spencer was only 20 years old when she married Prince Charles and became the United Kingdom’s own Princess of Wales. To the 750 million viewers who watched her 1981 wedding on television, Diana was a charming, down-to-earth princess who’d won her fairy tale marriage. But a decade later she sadly described the wedding as “the worst day of my life.”

Princess Di, as she was affectionately known, made that confession during a series of taped interviews with journalist Andrew Morton, who was writing a book about her life. She spoke candidly about her troubled marriage, her struggle with bulimia, and her difficulty handling life in the public eye. Most of the material has never been broadcast, but viewers can hear the interviews in the National Geographic documentary Diana: In Her Own Words.

Diana: In Her Own Words Trailer From award-winning filmmaker Tom Jennings, "Diana: In Her Own Words" is an intimate story told entirely in the Princess's voice through extremely rare recordings, most of which have never before been broadcast.

The film came 20 years after Lady Di’s shocking death in a car crash. Although by that time she had divorced Charles, she remained an international icon and mother to two princes.

We spoke with Tom Jennings, executive producer of the new film, about the humanitarian causes Lady Di championed and how she changed the role of the royal family. (See “Queen Elizabeth's Record-Breaking Reign in 14 Pictures.”)

How did the world react to the news of her sudden death?

When Princess Diana died, the whole world stopped. It was a remarkable and very sad series of events. The funeral was broadcast live around the world. People in the U.K. were putting flowers at the gates of Kensington Palace, so much so that for hundreds of yards you couldn’t even get to the palace gates.

Was the public always so infatuated with her?

People were fascinated by her to begin with. We grow up with this fairy tale fantasy of princes and princesses and castles, and all of sudden you had this very beautiful, very young woman living it. She almost instantaneously became more popular than Prince Charles. (Read “Prince Charles's Newest Cause: Combating Ocean Trash.”)

Into the 1990s the pressures of her marriage started to weigh down on her. After Morton’s book came out, she became kind of a polarizing figure. Some people thought that a royal should not be talking in public about what’s going on in their private lives. But other people who believed in her and who truly loved her became even more staunch supporters of her.

Her humanitarian work was also divisive, particularly when she shook hands with AIDS patients in 1987.

There was this unproven fear that if you just shook the hand of someone who had HIV or AIDS you would contract the disease, and Diana believed that not to be true. The people who loved her at that time loved her even more because here was a very public figure willing to show the world that these individuals need attention and care too. But some grumbled that a royal person shouldn’t be doing that. (See Remembering Diana: A Life in Photographs.)

Also during that time in the late ‘80s she took on homelessness, especially in the U.K. There’s footage of her going around and talking with people who are homeless, living in tents under bridges—and a lot of people just couldn’t believe it. She made a big point of saying that in the modern world people shouldn’t have to live like this. Right before she died, she took on the issue of landmines in war-torn countries in Africa, specifically Angola.

There was a conscious decision made on her part: ‘If I’m going to have cameras pointed at me the whole time, I might as well use all this publicity for good.’ And that’s what she did. She knew that her going out to hospitals or to fields filled with landmines would immediately draw the world’s attention to the problem. (Read “Meet the Giant Rats That Are Sniffing out Landmines.”)

How did her humanitarian work and willingness to talk about her personal life change the royal family?

Princess Diana had a tremendous impact on the royal family, and the people of the U.K., and their opinions of what the royal family meant to them. I think it carries on most significantly with her two sons, the princes William and Harry. She even says in our film toward the end that she’s altering the monarchy in a subtle way—for especially William, the older of the two—by doing all of the things that she’s doing, by taking on causes that the royal family would not normally take on.

Some people give her a lot of credit for modernizing the royal family by making it more engaged. It certainly has taken on a much more modern spin, and that modern spin started with Princess Diana. No one else had changed it quite as much as she did before then. And I think it is seen now as a much more accessible British institution. The two princes are often out interacting with the public in a way that Diana did. She was never afraid to go and shake hands with people.

This interview has been edited and condensed for clarity.