“BLACKFISH”, a gripping new documentary about killer whales in captivity, feels like an action thriller. Opening with the death of Dawn Brancheau, a trainer killed by a whale at Seaworld in Florida in 2010, it builds suspense with a haunting score, shock revelations and emotional footage of the killer in question—a sorry-looking orca called Tilikum. This film (now available on DVD) is an example of a new breed of theatrically-minded, more commercially viable documentaries that are contributing to the genre’s increasing success. Recent British Film Institute data show that the number of documentaries released in British cinemas has grown steadily every year over the last decade, from a measly four in 2001 to 86 last year. Documentaries now also account for about 16% of the Cannes film market according to its director, Jerome Paillard, compared with 8% five years ago. Netflix, an online streaming service that also makes television series, recently announced that it will soon be producing documentaries for the next wave of its original content drive.

The numbers may be small but they are growing. In 2011 documentary films grossed £11m at the British box office. This was only 1% of the year’s total box-office takings but it was a six-fold increase on the year before. Moreover, while the budgets can be high they are still much cheaper to produce than studio features.

But why are audiences increasingly choosing fact over fiction? Perhaps the current dearth of realism (endless comic-book sequels and apocalyptic action movies) is forcing more discerning viewers to choose authentic storytelling over spectacular visuals and far-fetched plots. Documentaries such as “Blackfish” may also fill a gap in investigative journalism, as fewer newspapers and broadcasters invest in long-term projects.

Anna Godas, chief executive of Dogwoof Pictures, Britain’s leading documentary distributor, points to a shift in film-makers’ attitudes. "Since the success of films like Michael Moore's “Fahrenheit 9/11”, which won the Cannes Palme d'or in 2004 (unprecedented for a documentary), and Morgan Spurlock's “Super Size Me” in the same year, I think film-makers have become more aware of how to make documentaries entertaining,” she says. Instead of picking worthy social causes and expecting people to be interested, film-makers are choosing charismatic central characters with naturally dramatic stories and presenting them in innovative, memorable ways. Directors such as Werner Herzog and Wim Wenders have also brought documentaries to wider audiences.

Two recent releases stand out: Joshua Oppenheimer's “The Act of Killing”, in which ex-members of Indonesia’s 1960s death squads, who are now active statesmen, are encouraged to re-enact their torture methods in the manner of their favourite Hollywood movies. And “Stories We Tell”, directed by Sarah Polley, a personal family drama made with archive footage (shot on Super 8 film) and interviews with her relatives.

The key to making such films cinematic, argues Mr Oppenheimer, is not by forcing a theatrical structure or an unusual format onto a documentary arbitrarily, but in finding what he calls the characters’ "true cinematic moments". These can then be sculpted into a compelling narrative during the editing process. “People watch documentaries for the drama of real people and the form can't feel forced," he says. Documentarians that try to make their films theatrical in structure end up with “still-born Hollywood imitations”.

Mr Oppenheimer claims that re-enactments in “The Act of Killing” were designed to encourage genuine drama. He wanted to capture the occasional flickers of doubt when a character questions the morality of his actions. In "Stories We Tell" the relatives' contradicting stories transform a forgettable detective tale into a complex meditation on memory. Its purpose seems to be to reveal the vulnerabilities of storytelling, and of Ms Polley herself.

Ms Godas believes that correct marketing is essential. "You have to capitalise on the film's intrinsic drama,” she says. “When we released “Restrepo” we could have pushed it as this sad investigation into what it means to be a solider in Afghanistan. Instead we pitched it as an action film, which is by far the more engaging angle. The same goes for “Blackfish”. It is a thriller in many ways." Launch budgets for some documentaries can now be more than £1m, she adds, a figure which has only emerged in the last few years.

Once marginalised as the dull work of worthy film-makers, documentaries are increasingly appealing to the mainstream. As the adage goes, truth is stranger than fiction.