How are great films made? They start with a great story.

In the case of the Ken Burns film, "The National Parks: America’s Best Idea," that story was written by Dayton Duncan. Burns and Duncan have collaborated on numerous films, from a documentary about the Lewis and Clark expedition to a biography of Mark Twain. The pair met in the early 1980s when Duncan was chief of staff for the governor of New Hampshire and Burns had an assignment to interview the governor. As Duncan recalls, "We became friends and recognized that we both had an interest in the story of America and in telling the story of America."

I had the pleasure of interviewing Duncan earlier this year when PBS re-aired "The National Parks: America’s Best Idea" in honor of the 100th anniversary of the National Park Service. Following are excerpts from that conversation. (Content has been edited for length and clarity.)

What inspired you to make a documentary about America’s national parks?

My awareness of the parks started when I was 9 years old. I grew up in a small town in Iowa. We took our first (and only) real vacation by borrowing my grandmother’s car and some camping equipment and heading west to visit national parks. That trip was exciting for a kid who hadn’t been outside of south-central Iowa much. I saw geysers and mountains and moose and bears and Devil’s Tower and deserts and all sorts of things, and it really opened my horizons.

Many of the films Ken and I made centered on uniquely American persons (Mark Twain, Thomas Jefferson), quintessentially American events (the Civil War), and American inventions, if you will (baseball, jazz). And I got the notion: Why haven’t we done one on the national parks? Those are an American invention as well.

We were the first nation in the world to say that our most magnificent, majestic, and sacred places should not be the exclusive domain of royalty or the rich and well-connected — they should be available to everyone and for all time. That was an American idea. It was the Declaration of Independence being applied to the landscape.

While it’s true that a national park has to be created by an act of Congress, if you turn over the "rock" of any national park, what you find is one or two people or a group of people who were passionate about the place and primarily responsible for saving it from being overly commercialized or privatized and for making it available to all of their fellow citizens.

We didn’t want the film to be just a travelogue. We didn’t want it to be just a nature film (although obviously nature is an important part of the parks story). We saw this as part of our constant investigation into who are we as Americans. What makes us tick? What things distinguish us? We wanted to use the parks as a way to continue that investigation.

Why are our national parks "America’s Best Idea"?

It is an idea made manifest. And that idea is linked with who we are as a nation, with those noble, aspirational words in the Declaration of Independence. A national park is both a real place and the application of that very democratic idea that is our nation at its best.

What does having the right to life, liberty, and the pursuit of happiness mean? That’s part of an ongoing discussion every generation has to settle for itself and for the future. The national park idea is a similar one. What constitutes a park? What should we be able to do in them? Why do we have them? All those things are part of the American story.

It’s the story of people who fall in love with a place so completely that they decide to dedicate themselves — often for many years and often against great opposition — to set that place aside so people they would never meet and generations they would never know would have the chance to experience that place just as they did and fall in love with it too.

Why should Americans today care about our national park system?

One early congressman called national parks "great breathing lungs for the national spirit." They’re a place where you can go and revive your health and spirits. Sometimes you can find — as John Muir did — a connection to things that are larger than yourself.

The first director of the National Park Service, Stephen Mather, called the parks "vast schoolrooms of Americanism." By which he meant that someone going to a national park would so appreciate it that they would also have a greater appreciation for the nation that set that aside. So the parks would make us better citizens.

We now recognize that they are also vast schoolrooms — whether it’s learning about the American story at historical sites or learning about science at places where you can encounter the natural processes of flora and fauna.

For some people, parks are the places where they can go for extreme sports. Other people might step off a tour bus at the Grand Canyon and stand there for 20 minutes and stare at that yawn space — a mile deep and 10 miles across with millions of years of geological history — and think, "That’s the most beautiful thing I’ve ever seen," then get back on the bus and move on. There’s nothing wrong with that either.

What do you want people to take away from "The National Parks: America’s Best Idea"?

We would hope that people would take away several things:

National parks are uniquely American. They are something we should be proud of as a nation and, in being proud of it, commit ourselves to making sure they will be protected in the future.

Just as parks relied on the dedication of individuals and small groups of people for their creation, they still rely on individuals and groups to make sure that gain is not lost. Once a place is ruined, it’s ruined. Once a place is saved as a national park, it’s only saved for as long as it continues to be saved. Our series shows that there have been threats to the parks off and on throughout the history of national parks.

An invitation to go and see these places themselves. We’re very proud of the film and we think it’s got some incredibly stirring and emotional stories in it. It’s educational and has the best cinematography we’ve ever done. But it doesn’t compare with actually going to the national parks yourself.

What are the challenges and opportunities for the future of the park system?

Every American is an equal owner of the greatest collection of geysers on Earth, the grandest canyon in the world, the tallest mountain, the biggest trees — regardless of their station in life, how much money is in their checkbook, how long they have been in the United States.

The greatest challenge facing the parks and the National Park Service is one that they recognize very well: In an increasingly urban, increasingly diverse, increasingly technology infatuated, couch-sitting population, to ensure more and more people understand that they are welcome and that there’s something of value for them in the parks. For a variety of reasons, for much of the history of the national parks, a number of groups did not feel that they were welcomed there or that it had any real relevance to them. Once you get them there, they almost automatically become greater park champions.

We’ll never run out of people in the United States who, if they look down into a big canyon with a river in the bottom of it, will say, "Hey what a great place for a hydroelectric dam." Or someone who will look at a beautiful valley like Yosemite and think, "Golf courses in the meadows and trophy homes and a gate." Or who would look at a swamp in southern Florida and think, "If we could just drain this, we could have shopping centers and housing developments." Or look at a mountain and wonder what kinds of minerals are probably there. Or look at a forest and immediately calculate how many board feet of lumber you can get out of it. That’s part of who we are, both as human beings but also particularly as Americans. The challenge is to make sure that we also work to have new generations of people who say, "No. At least here, you can’t do that."

How do you get youth to unplug and get to the parks?

One thing we found when we investigated the history of the national parks is that nothing’s easy. It always takes time, perseverance, dedication, and recognition that something needs to be done. The Park Service is working hard to diversify its workforce and working with partners to use the parks as classrooms to get younger people to have some connection with a park or historic site or whatever. It takes works.

I think the Park Service and its partners recognize that one of the chief challenges and opportunities for the future is creating a new generation that reflects our nation who have encounters with — and therefore become champions of — the national parks.

What did you take away from meeting the men and women of the National Park Service?

I have the greatest amount of respect for them, and I think that is shared by many Americans. I also think they have a tough job of trying to balance the notion of places we want to encourage people to visit with having rules to ensure those places are still there and still special for the next generation.

What is your response when people say the "federal government" owns too much land?

There are more than 300 million co-owners of that land. And we live in a democracy. If you have a good argument that you think can persuade a majority of our citizens, there’s place where that discussion happens. It doesn’t happen at the end of a gun. It doesn’t happen by going in and taking over a place. It ends by changing the law.

Which national park is your personal favorite and why?

My favorite would be Glacier National Park. It’s got these incredibly beautiful mountains, lots of cascading streams and waterfalls, and crystalline lakes. You go there and you’re likely to see bears, moose, Rocky Mountain goats, bighorn sheep. And if you go there soon, you still might see a few glaciers! It’s a beautiful spot.

The reason it’s my favorite, however, is because it’s the place Dianne and I visited together (before we were married) and it became the first park we took our two children to. Over the years, we built a lot of special memories there. So it has that special place in my heart.

For more information about "The National Parks: America’s Best Idea," including excerpts and educational resources, visit www.pbs.org/nationalparks.