I remember, vividly, hearing the news of Marie Colvin’s death: I was in the staff canteen, about to eat lunch, when a breaking news alert flashed up on the TV screen. I couldn’t believe it: I’d been listening to one of her news reports from Syria, just the night before – and had marvelled at the bravery of journalists and photographers who place themselves in the hideous situations in order to report the truth.

It’s six years since the world of foreign correspondents lost one of its most respected and colourful reporters. This month, her friend and fellow correspondent Lindsey Hilsum is publishing a biography of Colvin called ‘In Extremis’. “We weren’t best friends”, she clarified tonight, at the Dulwich Literary Festival. “This book would have been too painful to write had that been the case”. Yet the two women knew each other well and Hilsum thought of them as the “Thelma and Louise” of the press corps.

It’s hard to know where to start when talking about Marie Colvin, whose private life was “as much a battle zone as those she reported on”. Let’s begin, then, with Hilsum’s decision to write this book. “Paralysed” into inertia for the year following Colvin’s death, Hilsum refused to visit any dangerous zones. But, while recovering from a knee injury, she began thinking about Marie Colvin and her life.

On a visit to Long Island, where Colvin was born and grew up, Hilsum was astonished to find that Colvin had left behind over 300 journals and notebooks, chronicling her life since childhood. Lent them by Colvin’s mother, Hilsum read each one, even breaking a locked, plastic diary kept by a 13-year-old Marie. “She was a very naughty teenager”, Hilsum chuckled, reminiscently.

Her teens saw Colvin, the oldest of five children in a devoutly Catholic family, protest against the Vietnam War, open a recycling facility at her parents’ home, without their knowledge, chase after all sorts of unsuitable boys – and tell the occasional fib. “But only in her personal life: never in her journalism”.

At Yale, Colvin studied non-fiction writing under the legendary John Hershey, a former WWII correspondent and author of ‘Hiroshima’, considered by Colvin to be the best book on war ever written. It was this experience that inspired her choice of career.

Hilsum interviewed 114 people who had known Colvin, from her school friends to the wife of the late Yasser Arafat. “Marie was an extraordinary journalist”, Hilsum recalled, “with an incredible eye for detail”. Although often profoundly shocked by what she saw, Colvin was never immobilised – and always got closer to the action than any of her peers.

Who else would have remained in East Timor when all other journalists had been ordered out by their editors? In a compound Colvin stayed, with two female Dutch journalists and a core of female UN staff, cracking jokes about how “they don’t make men like they used to” – which wasn’t completely fair, grinned Hilsum, given that two male journalists were hunkered down in the surrounding hills.

And yet, Colvin embraced uniformity from time to time. She married her first husband, Daily Telegraph correspondent Patrick Bishop, wearing her mother’s wedding dress and accompanied by six bridesmaids wearing “hideous” Laura Ashley dresses (they’ve never forgiven her, so one of them told Hilsum). The marriage itself wasn’t conventional, however: Patrick cut short their honeymoon to fly to Beirut and report a story – but also, unbeknown to Colvin, to see his French girlfriend, who was on assignment in the Lebanese capital.

Colvin’s second husband, Bolivian journalist Juan Carlos Gumucio, did not treat her any better. He was “crazy and a cokehead” – but the couple were happy together in war zones, where real life was suspended. It was when they were dealing with the mundanity of everyday life in London that the problems began. The couple eventually separated and Colvin remarried Patrick Bishop.

Marie Colvin wanted to be where history was happening and she had a knack for sniffing out a story. Following the American bombing of Libya, she was the first journalist to secure an interview with Colonel Gadaffi – a “predatory” individual who insisted on conducting the interview in a dingy basement. Despite his “deeply menacing” attitude, Colvin persevered – and the interview made headlines around the world.

But Colvin understood the risks of war reporting. She wanted to report what people were going through – not the number of bombs dropped or the types of tanks driven. This fearless woman did have a fragile side, though: she was great fun on the road but, according to Hilsum, there was a lot of sadness underneath. Much of this derived from the horrendous PTSD with which Colvin was diagnosed after she lost an eye in SriLanka. She found living with one eye “very traumatic”, as she could no longer take for granted everyday actions, and reading took huge effort. Plagued by terrible nightmares, Colvin had to adjust to the fact that something irrevocable had happened.

It’s perhaps not surprising that this pioneering female correspondent (and cherished mentor of younger journalists) smoked and drank too much – and chose men who couldn’t make her happy. “Flawed and fragile”, she had become like her heroine, Martha Gellhorn. Despite everything she went through, however, Colvin remained utterly committed to journalism and her belief that what she did for a living made a difference.

That belief is what found Colvin in Beirut in February 2012, having dinner with Hilsum and two other journalists, discussing the plight of Syrian refugees. All three of her dining companions told Colvin that her plan to smuggle herself in to Syria was too dangerous, but she went ahead anyway, undertaking a deeply arduous journey with photographer Paul Conroy. From there, she published an “extraordinary” story revealing the truth about the Widows’ Basement and destroying the Syrian government’s claim of having been unaware it was shelling women and children.

Mission accomplished, Colvin left Syria – only to return, suffering from survivor guilt. When Hilsum found out: “I was furious. I rang Marie to tell her what I thought, but all she would say was that what she was witnessing was the worst thing any of us had ever seen”. Exasperated, Hilsum asked her friend whether she had an exit strategy. “Not yet – but I’m working on it”, Colvin replied. Hours later, she and French photographer Remi Ochlik were dead, after a rocket struck the rebels’ media centre (Paul Conroy was seriously injured but eventually escaped).

Lindsey Hilsum is too experienced a journalist to betray much emotion, but there’s no doubt that Marie Colvin’s death has affected her greatly. To hear her speak so eloquently about her friend and colleague was a privilege and I intend to read ‘In Extremis’ as soon as possible.