One of the key attractions for cineastes at this month’s New York Film Festival is The Dog, a documentary inspired by the 1975 classic Dog Day Afternoon starring Al Pacino – one of the great New York movies of the 70s. The Dog is an exploration of the real-life Brooklyn bank robber John Wojtowicz, famously portrayed by Al Pacino in the picture. Wojtowicz maintained that he staged the thwarted bank heist because he needed money to fund his boyfriend’s sex-change operation. The robbery became a televised spectacle with a dramatic standoff between the police and Wojtowicz, who was holding hostages.  The Dog, which was launched at the Toronto Film Festival earlier this month, belongs to a growing group of documentaries inspired by feature films.  Documentary filmmakers have long looked to landmark movies as source material. The 1982 documentary Burden of Dreams focused on director Werner Herzog’s chaotic but successful struggle to make the movie Fitzcarraldo. In that film the director arranged for a 320-ton boat to be pulled over a hill in the Amazon jungle.  The 2002 documentary Lost in La Mancha memorably captured Terry Gilliam’s doomed first efforts to make The Man Who Killed Don Quixote. The production, which was to have starred Johnny Depp, was beset by all kinds of problems including bad weather and illness.  More recent additions to this documentary sub-genre include the award-winning Room 237 which examined the theories of obsessive fans of Stanley Kubrick’s classic 1980 horror film The Shining and their different interpretations of his work.  Also showing at Toronto this year was Jodorowsky’s Dune which documents the efforts in the 1970’s to adapt Frank Hebert’s bestselling sci-fi epic Dune into a feature film. At the helm of this endeavour was Chilean-born director Alejandro Jodorowsky ,whose credits included the surreal 1970 western El Topo, and 1973 cult classic The Holy Mountain produced by The Beatles manager Allen Klein.  Dune was a very ambitious undertaking – there was talk at one point that it would be fourteen hours long. “For me,” declared Jodorowsky, “Dune will be the coming of a god. I wanted to make something sacred, free, with new perspective. Open the mind!”  According to Thom Powers, who programmed the documentaries at Toronto this year, the often grand plans of film directors – and the risks they inevitably take on – provide non-fiction filmmakers with compelling material.  “People used to sail ships across the ocean with a lot of money at stake and a lot of risk at stake,” says Powers, “and filmmaking is that same kind of adventure. You cast off with a prayer and a hope. You’re hoping that the captain is going to lead you to something at the other end that represents adventure and perhaps financial gain. That is great subject matter for making a film.”  But of course not every grand film can sustain a documentary. There has to be something unique by way of content, filmmaking and personalities.  For Rodney Ascher who directed Room 237, it was Kubrick’s sheer artistic mastery with The Shining that made the film such a source of fascination for movie fans and for him as a director. Few would deny that The Shining has a potent spellbinding effect that really lingers.  “There’s something about The Shining, the techniques that it’s made with, the compositions are so beautiful and so iconic, so many of the images stay with you,” says Ascher.  And with Jodorowsky’s Dune, the director’s vision was so expansive that it was ripe for scrutiny. “When you look to the history of films that fell by the wayside, films that didn’t happen,” says Frank Pavich, “to me, Alejandro Jodorowsky’s version of Dune is the most exciting of all of those. His is the one that had Pink Floyd and Mick Jagger and Salvador Dali.”For the documentary maker, it is “the greatest of all unmade film projects.”  Many film-inspired documentaries require extensive research and legwork. Often the projects are a real labour of love. Relationships need to be formed; reluctant interviewees need to be coaxed to talk on camera. With The Dog a big effort was made to get participation from John Wojtowicz’s former friends, lovers and family members. As Allison Berg who co-directs The Dog recalls: “We began the film about 11 years ago, and it’s been a really long road. We became so attached to the people we were filming, relationships were formed.”  Berg and co-director Frank Keraudren became so consumed by the story that they went to visit participants in the film without the goal of shooting. “A lot of times that we weren’t actually going there with a camera, but it was just more about the life experience of doing this,” she says.  There is always the danger with documentaries rooted in feature films that the content will only be comprehensible the passionate fans of the movies under examination.  Jodorowsky’s Dune’s director Frank Pavich says that “in order to watch Room 237, I think you need to know The Shining, at the very least.” But he sees Alejandro Jodorowsky, now 84, as having such an appealing, optimistic personality that will win audiences over to the film. “I don’t think you need to know his previous films and I don’t think you need to know about Dune. If you like people and you want to be inspired, I think that’s all you need,” says Pavich.  Documentaries based on feature films can lead audiences back to revisit the original works. This is almost certainly the case with Room 237. The film has been shown at some film festivals in tandem with special screenings of The Shining.  Allison Berg is hopeful that The Dog will prompt people to watch Dog Day Afternoon one more time. “I think they work really well together,” she says. “I think it’s going to be a lot of fun in terms of the back story. I was very satisfied when I learned all of that. I think that they’ll enjoy it.”  This trend of documentaries inspired by feature films won’t dry up. They’re enough fascinating characters in movies based on real-life figures like John Wojtowicz to warrant an ongoing investigation by non-fiction filmmakers. And there are a sufficient number of directors with colourful and grandiose schemes to satisfy a whole procession of documentary filmmakers wanting to chronicle their adventures. It’s a sub-genre of documentary filmmaking that looks set to continue.  If you would like to comment on this story or anything else you have seen on BBC Culture, head over to our Facebook page or message us on Twitter.
A raft of new documentaries tell the stories behind famous movies. Tom Brook explains why directors’ grandiose schemes are so fascinating to other filmmakers.