Since her widely acclaimed first feature Devil’s Playground debuted at Sundance in 2002, London native Lucy Walker (one of Filmmaker’s 25 New Faces of Independent Film that year) has distinguished herself as a resourceful documentarian with a discerning eye for character detail. A study of Amish adolescents sampling the forbidden fruits of the modern world during “rumspringa,” an elective time spent away from the strictures of their traditional religious community, Playground was an insightful, humanizing portrait of a little-seen, faintly understood social milieu. For her follow-up in 2006, Blindsight, Walker again took on an uncommon challenge, trailing a group of sightless Tibetan teens attempting to scale the treacherous Lhakpa Ri peak of Mount Everest under the more experienced guidance of a blind climber. Even prior to venturing into documentary, and not long after she’d left NYU’s graduate film program, Walker’s talent was already apparent, as she earned two Daytime Emmy nominations for her work on the children’s TV program Blue’s Clues. More recently, at Sundance 2010, Walker was one of the rare filmmakers to unveil two features simultaneously at the festival. Waste Land is a profile of renowned artist Vik Muniz as he creates art out of garbage at the world’s largest landfill on the outskirts of Rio de Janeiro, collaborating with other trash pickers. While this film, winner of an Amnesty International Prize at the Berlinale in February, merits all the praise it has received, her other equally compelling new doc, Countdown to Zero, has an urgency that cannot be ignored.

The threat of nuclear annihilation may seem like a relic of the past, a Cold War hangover that lingers in the minds of first-strike warmongers or high-strung, atavistic nuts, but Countdown to Zero (produced by An Inconvenient Truth‘s Lawrence Bender and funded by Participant Media and World Security Institute) makes a very compelling case for why disarmament should be at the forefront of our geopolitical thinking today. Speaking with an impressive roster of nuclear scientists, military strategists, authors, physicists, and former heads of state—including Pervez Musharraf, Tony Blair, and Mikhail Gorbachev—Walker outlines a number of frightening scenarios that could lead to a catastrophic event, and some that already have. The threat of terrorism is foremost in her mind, as the availability of enriched uranium on the black market and its virtual undetectability in shipping containers make the possibility of a dirty-bomb attack in a major Western city seem inevitable. Scarier still are the near-miss incidents recounted here: In 1995, Russian president Boris Yeltsin had to make a split-second decision about whether or not to launch a full-scale nuclear attack on the U.S. when research rockets sailing high over Norway were mistaken for incoming missiles. B-52s carrying nuclear warheads have crashed over both Greenland and South Carolina. Combining archive and graphics with sobering accounts of the triple threat, keyed to John F. Kennedy’s pronouncement at the U.N. General Assembly in 1961 that a large-scale nuclear event could result from “accident, miscalculation, or madness,” Countdown to Zero is a button pusher with a nervy yet noble agenda: saving us from self-extinction.

Filmmaker sat down to speak with Walker about nightmares, verité moments, low-probability events, and how to nail a world leader for an interview. Magnolia releases Countdown to Zero in New York on Friday.

Filmmaker: Is it just because the Cold War came to an end that the threat of nuclear devastation quietly slipped off our radar?

Walker: Who wants to worry about nuclear horror? I think complacency has set in. Nothing catastrophically bad has happened since 1945, nothing that we’re all afraid of. We were desperate not to have to worry about these things, but looking at the evidence, I think that was a terrible mistake, because the threat’s gotten progressively worse since the end of the Cold War.

Filmmaker: Watching the film, I was surprised at how many people don’t seem to know how many nukes there are in the world, or which countries even have them.

Walker: Yes, there was an amazing study in 2004 that said Americans thought it was more likely that aliens would invade than a bomb could go off! People are outrageously ignorant about these things.

Filmmaker: Did your motivation for making Countdown to Zero stem from a preexisting concern with the subject?

Walker: My worst nightmares as a child were about nuclear weapons. Documentaries are a pain to make, and there’s no point unless you really believe something urgent needs to be said. I felt this was the most urgent issue facing our species, quite literally. So as a filmmaker, that’s the kind of challenge I like.

Filmmaker: In Blindsight and Devil’s Playground, and now more recently with Waste Land, your films have been very character-driven. But this one’s different.

Walker: Yeah, but I actually tried with this one to find characters. It was really challenging because most people don’t let you film much about nuclear weapons because it’s all so sensitive, trying to follow somebody or get into character arcs or something more narrative. I was embedded in Budapest, and went camping in Kazakhstan. I really toiled to try to find compelling verité moments. It would have meant sneaking around the back and going to play with one of those things, you know, and setting it off. So thank goodness, right? It’s really constraining, because it’s me and a cameraman carrying all the equipment through customs and I’m doing sound and booking travel since there’s no budget for an assistant, you know what I mean? As a filmmaker, you’re wearing every hat. And even when you get one of these world leaders, which is tough to line up, you only get a few minutes and you’re trying to make them say something interesting. [Laughs] Every way you look at it, it’s a challenge.

Filmmaker: At what point did you decide you needed to shift your approach, that you weren’t going to get those verité moments you were searching for? Were you comfortable with that?

Walker: I think with every movie, you’re always turning over every stone. There’s a giant committee on this movie and a big campaign in the film – I didn’t originate the project, it wasn’t like there was a treatment – I wasn’t even mandated to come to any conclusion. It was just: “Nuclear weapons …go!” [Laughs] At that point, I had to educate myself because I’m not an expert – I’m a smart person and a good researcher and a hard worker, but I needed to make sure I would fulfill this awesome responsibility. It was terrifying, to be honest. So I was determined to be intelligent, I suppose. Initially, when I was doing my research, I’d already met Gorbachev and a lot of nuclear scientists, and I knew we were going to have permission to interview them for the film. But in the end it took years. I had to meet Gorbachev three times in different cities before I could actually film these meetings, when I was finally in Moscow. As a filmmaker, I’m just obsessed with trying to tell the most dramatic, emotionally engaging story that I can.

Filmmaker: All of that work led you to somebody like Oleg Khintsagov, the smuggler who sold nuclear materials to an undercover agent posing as an Al Qaeda operative.

Walker: No print journalist even had ever managed to speak to him: I got three hours on camera. But in order to secure that interview, I had to get permission from the American agencies, Georgian and Russian governments, local prison officers, and Oleg himself. It was just a lot of work. The amount of work that didn’t bear fruit was tremendous. For every Amish kid that says yes, there are hundreds who say no. We were embedded on this incredible mission in Budapest and it all went wrong, it was like a thriller. A train that was supposed to be securing seven nuclear weapons’ worth of material got turned around at the border of Slovenia because there was an election and the prime minister of Ljubljana decided it was too sensitive to let the nuclear material go through to the port, where it was going to be shipped to Russia. Hungary didn’t want it back because it was no longer securable. It was a total John Woo movie premise, with phone calls going back and forth between prime ministers and ambassadors and everybody flipping out because this is the stuff of horror movies, and I wasn’t allowed to film a thing. [Laughs]

Filmmaker: I was shocked to learn of the near-miss with Yeltsin and the B-52s crashing in Greenland and South Carolina. What did you discover in researching this film that made you most afraid?

Walker: All of it. You know it’s possible, but you don’t think those possibilities could add up to an actuality, and unfortunately every single person I spoke to said, no, it’s real. I looked for reassurance, I really looked for a solution [to the problem] that a nuclear catastrophe is a matter of when, not if. I didn’t want to have to draw that conclusion. Yet nobody could give me a sensible counterargument. And nobody could tell me that the steps needed to be taken to blow up New York City not only could happen but had already happened. That was really shocking to me. You want to think nuclear weapons secrets aren’t smugglable, you want to think uranium isn’t enrichable very easily. You want to think no stuff actually gets loose. Who would have thought kitty litter was more radioactively detectable than fissile material? Isn’t that insane? Scott Sagan [The Limits of Safety], this guy who theorizes about accidents, really helped me think about and understand that low-probability events happen all the time.

Filmmaker: Did you sense ahead of time that all your experts were essentially going to reach the same conclusion?

Walker: Everyone argues massively on the finer details, like with any community. There was a good question about whether to include counterarguments, but I felt a bit like I did with An Inconvenient Truth, that for me they aren’t very credible. Nobody could convince me that a world where many states have nuclear weapons, and nonstate actors can shortly have them too, is a safe world. I spent a week with the IAEA [studying] the science of verification and why it is, from the scientists’ and international inspection-regime point of view, that the less nuclear weapons there are in the world, the easier it is to control rogues.

Filmmaker: You even spoke to A.Q. Khan, the shadowy entrepreneur who helped develop Pakistan’s nukes.

Walker: I had amazing interviews with him on the phone talking about the Pakistani nuclear weapons program. Some things were too sensitive to make the cut, and as a filmmaker, you regret that, because when you do get the dynamic footage, it’s hard not to want to include it.

Filmmaker: I’m sure a lot of filmmakers would be interested in knowing how you spoke to so many heads of state. How does that work?

Walker: Usually with a lot of letters. We had letters and we had an urgent threat [to discuss]. When you’re aligned in a really powerful way and your intentions are really good, nothing is more helpful. There was no tricky business, we were incredibly straightforward and honest. Something I’ve always been proud about with all my movies is that no one’s ever regretted being in them. Given how sensitive the topic is, there were leaders who chose not to participate—there were many who turned us down. As a world leader, imagine how hard it is not to say the wrong thing. [Laughs] It was Bob McNamara’s last public interview, and he felt so motivated about the issue. I made a beeline and got that interview first because he was fading from life. That was an honor. I wanted to feel confident that audiences would understand the issue. I wanted to give people the tools, if they were reading about the Iranian nuclear weapons program or the North Korean missile tests or the instability of the Pakistani regime, to understand what that actually means.

Filmmaker: What motivates you as a filmmaker, showing people something they haven’t seen before or haven’t thought much about, or changing people’s minds?

Walker: The theme of changing people’s minds was really prominent for me. I thought a lot about this Einstein quote, in which he called nuclear weapons his one mistake, and said that they’d changed everything apart from our way of thinking. And my manifesto was, I don’t want to make a doc about how NYC was destroyed by a nuclear bomb, I want to make a film that stops that happening. I love this quote by Reverend Richard Cizik in the movie [where he says] if you haven’t changed your mind about something, pinch yourself, you may be dead. I feel like we are in a new era and Cold War thinking, that whole paradigm, is no longer extant. We really need to update our thinking on this issue.