Every once in a while, a culture shifts. You feel like a Luddite until your new learning curve is complete. That is the experience I have been having recently, as my book The End of America has been turned into a documentary. Can political documentaries make a difference? For someone who lives mostly in the dimension of words, it is an exciting and scary question. The End of America details the 10 steps that would-be dictators always take in seeking to close an open society; it argued that the Bush administration had been advancing each one. I took the message on the road, and one of those early lectures — at the University of Washington in Seattle, in October 2007 — was videoed by a member of the audience. Even with its bad lighting and funky amateur vibe, this video, posted on YouTube, has been accessed almost 1,250,000 times.

This was a humbling lesson. While a polemical argument in prose may reach tens of thousands of the usual suspects — formally educated people who like to follow such texts — the video version reached far beyond that audience. Everywhere I went, from the gas station to the nail salon, I ran into people who would have been unlikely to read a book of mine, but who were passionately supportive of the argument from having watched it on YouTube. The medium really is the message, in this case. For any opposition to Bush’s assault on liberty to be real, we would need hundreds of thousands of Americans from all walks of life to become outraged. Many other videos and films helped reached those masses, including Taxi to the Dark Side, Alex Gibney’s documentary about US brutality towards terror suspects, which last year won an Academy award, at a time when the major newspapers were still queasy about calling Bush interrogation tactics torture.

My humbling experience of the limits of print was taken one step further by a team of documentary-makers, Ricki Stern and Annie Sundberg. As they worked on their film of The End of America, I experienced something incomparably fascinating for a non-fiction writer: sources I had quoted at length from the written record were interviewed in person, with all their humanity and quirkiness. Oh my God: there was Captain James Yee, the Guantanamo chaplain framed and held in a navy brig in isolation, because he spoke up against abuse of the detainees. There he was in his living room! Wow: here was Colonel David Antoon, the Vietnam veteran, fighter pilot and Iraq war critic, breaking down in tears as he described the harassment of his elderly mother. These voices came to life with surreal vividness.

I also saw the power of news footage, both archival and contemporary, to move the emotions in a way that my poor computer could never do. It is one thing to invoke in prose the history of how Pinochet rounded up citizens for violent intimidation, and another to see actual footage of people who look like you and me dragged off by the hair in modern city streets. It is one thing to analyse a militarised post-9/11 US police response to protesters, and another to watch never-before-seen footage of US police officers engage in mass sweeps of terrified citizens in St Paul, Minnesota, including parents with children, and dragging a frightened reporter, Amy Goodman, off by her sweater. Is this medium effective in bringing about change?

The history of documentary film is nearly 100 years old, and its tradition owes much to early documentarians in Britain in the 1920s and 30s. In the US, the 50s and 60s marked the documentary’s golden age, especially at CBS, where pioneering television journalist Edward R Murrow, immortalised in George Clooney’s Good Night, and Good Luck, produced such landmark investigations as the CBS Reports programme Hunger in America. Recent heirs to this tradition include such films as Al Gore’s An Inconvenient Truth — which, with his book, arguably did more to raise mass awareness of the reality of global warming than any amount of print analysis, or legislative debate, might have.

But remember: Gore brought out his film along with his book. For all the power of video and film, I am not giving up my pen. I am just much more likely to try to link essays to webcasts or videos. The best way for these two media to move forward, to inform and make change, is in tandem; together they are more than the sum of their parts. Documentary film without nuanced journalistic sourcing risks being sensational, tendentious or broad-brushed.

These days, print without a dimension of imagery risks being flat, especially to a younger audience. So while we need not despair about the future of investigative journalism, or the power of print alone to drive change, we writers should accept the inevitable: those damn film-makers have tools we need to adapt to, and, wherever possible, appropriate. Wherever we want to turn out the deathless prose of political polemic to drive great change — well, we just have to smuggle out the video footage to go with it. — The Guardian