When I contact the photographer Paul Conroy, he sounds chipper for a man who has spent a lot of the past six years recounting his last assignment with the renowned Sunday Times war correspondent Marie Colvin. They were together when she was killed in an attack by President Bashar al-Assad’s forces in Baba Amr, Homs, bringing to an end a partnership forged from a shared desire to “bear witness”, even if that meant putting themselves in danger.

Conroy escaped with a huge leg wound, and the urgent plea of some of his Syrian helpers to “go tell the world”, ringing in his ears. “I’ve been doing that as much as I can ever since,” he says.

After leaving hospital, he spent six months cathartically writing a book, Under the Wire.

“I went through every minute of that trip,” says the Devon-based Liverpudlian. “From when we landed to when I got out, and just lived it all again. There was nothing left, no little dark holes with cupboards. I went through everything.”

Eight years later, Conroy still feels a responsibility to keep talking about Colvin, his French photographer and friend Remi Ochlick, who died in the same attack, and the dire situation in Syria. “Trying to hold people to account for what happened, and what was allowed to happen. Not a day goes by when I don’t think, ‘What have I done today? What am I going to do today about this?’”

We’re talking because his book has been made into a documentary, also called Under the Wire, which combines new interviews with Conroy and other key people involved in the story, authentic-looking reconstructions, and archive material. The result is compelling and harrowing, not least because of the insight it gives into the human tragedy Assad is inflicting on Syrians. This was important to Conroy, and informed his decision to go with Arrow Media when they pitched their vision for the documentary.

“What I dreaded was some film where I end up coming out in a string vest looking like Bruce Willis at the end,” he says laughing. “The whole thing was never about me and Marie. We just went to do the job.”

Their approach to war was to report on it from “the sharp end of the stick”. They wanted to create an empathetic connection between reader and subject so that their stories didn’t just inform, but also carried the real possibility of making a difference. “[Marie] was convinced that she could save lives. In Homs, especially.” Thus in words and pictures they put people, not geopolitics and military hardware, first.

“You see them really nice pictures of cruise missiles going down chimneys, and it’s all very smart and clever, but the reality is completely different,” says Conroy. “Every time we’d go out to these places, you’d end up with the same people: the women and the children, hiding in basements, starving.

“What we always wanted to do was present their story; and to get that story you had to go to some pretty dreadful places.”

His first experience of this side of war came after he left the army (“Alas, me and the army didn’t see eye to eye”), and later joined friends on an aid convoy to Kosovo. He’d worked on some films with Willy Russell and took a camera along to document the trip.

As part of a heavy artillery unit on the East-West German border, his thinking about war had been dominated by the “USSR-Nato stand-off”. “It was nothing like I had been told about war, and trained for,” he says, after seeing people coming out of Kosovo. “I was intrigued. So when the aid convoy left, I stayed on.”

The seeds for his future as a war photographer were being sown. But he admits that it took an adjustment before he felt comfortable doing the job.

“Initially it was an awkward feeling because I was seeing these people come out and just etched on their faces was this kind of misery and pain I had never seen before, and it was really hard, initially, to step into that and put a camera in their face. There is a definite sense of intruding on someone’s grief.”

Conroy, though, noticed that when a whole village has been through a massacre or witnessed atrocities, it is hard for people to talk to each other about it. Therefore when journalists arrive, they “queue up to tell their stories”. “You soon realised that you weren’t intruding,” he says. “And I think that goes to the heart of why I kept doing it.”

The first time he saw Colvin in the flesh, in 2003, Conroy was in a room in northeast Syria with journalists waiting to be allowed into Iraq. “We had been there about a month,” he recalls. “She walked in, looked at everybody laying there on these couches and chairs, drinking tea, and just went, ‘Oh my God, they’re drugging the fucking journalists,’ and walked out. I thought that was wonderful.”

Unknown to anyone, Conroy was secretly building a boat out of inner tubes in his hotel room to make his own way into Iraq across the Tigris River. The attempt failed; he was detained, and told to leave Syria within days. His actions alienated some journalists: “They said I’d spoilt it for them by doing this mad boat-building exercise,” but Colvin was impressed. She came to a restaurant where he was sitting alone in a corner, and said: “‘Who and where is the boatman? And do you want a drink?’ I was, like, ‘Yes please’, and that was it. We soon realised that we wanted the exact same thing when we did the job,” Conroy recalls.

Under the wire: photos by Paul Conroy Show all 12 1 /12 Under the wire: photos by Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Rebel runs for cover from a 500lb bomb dropped near ammunition dump at at Ras Lanuf Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Four year old Tamin Adel being treated for shrapnel wounds at Al Hekma hospital after a coalition military intervention, Misrata Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Rebel soldiers pray before fighting begins at checkpoint near Ras Lanuf Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Journalists run for cover in Ras Lanuf Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 The charred remains of one of Gadaffi's loyalst troops after being incinerated in a NATO airstrike, Sirte Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Rebel soldier poses on bicycle at a checkpoint at Ras Lanuf Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 The storming of Gaddafi palace at Baba Al Azia Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Red crescent volunteers remove the bodies of Gaddafi loyalists who were killed when NATO attacked the convoy of Ghadaffi in the town of Sirte Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Rebel soldiers run for cover during mortar attack near Ras Lanuf Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Wounded children during a coalition military intervention Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Interior of family home damaged when a mortar hit during a coalition military intervention, Misrata Paul Conroy Under the wire: photos by Paul Conroy Libya – 2011 Rebel soldier takes a break during fighting in Ras Lanuf Paul Conroy

Colvin already had a formidable reputation. She’d lost an eye to a grenade in Sri Lanka in 2001, giving rise to her signature eyepatch, and in 1999 had famously refused to join other journalists as they poured out of East Timor. Her reports are credited with helping save the lives of 1,500 women and children who were besieged in a compound by Indonesian-backed forces.

Fiercely independent, she was notoriously difficult to match with photographers. According to Sean Ryan, a former Sunday Times foreign editor interviewed in the documentary, one even complained that Colvin was more frightening than the war they were covering. Conroy laughs. For him “it was effortless with Marie”.

“When we worked together, it really clicked. It was unspoken. We both knew what we had to do. There was never any conflict at all in what we were after.”

Getting what they wanted frequently involved huge amounts of personal risk, and neither of them got a buzz from war. If they were addicted to anything, says Conroy, it was to “the story”, not “the fear and danger”.

“I don’t like being shot at. I don’t really like sitting being shelled. It’s fucking terrifying. And Marie was the same. She had a deserved fear of the front line, and her greatest strength was the fact that she overcame it [and went back].”

Inevitably, the strain took its toll. Colvin suffered from PTSD, and Conroy always knew when she was in trouble.

“We’d be driving along and she’d go from chat, chat, chat to quietly staring out the window. I knew that’s when I needed to step in and either reassure her or crack a little joke, just to break that spell. Fear can consume you.”

There was plenty to fear about Homs in 2012. Assad regarded it as the beating heart of the revolution and had it under daily bombardment. In an attempt to hide what was happening, he tried to silence the media. The day before Conroy and Colvin went in, a contact in Lebanese intelligence informed them that any foreign journalists found in Baba Amr “would be executed and their bodies put on the battlefield, as if caught in crossfire”. In Under the Wire, the correspondent Lindsey Hilsum says Syria was too dangerous for her.

“I completely get it,” says Conroy. “We’ve all got different levels of tolerance. And with me and Marie, we weren’t gung-ho. We did it because we thought we could do it. Marie always used to say no story is worth dying for, because there’s no story then.”

They entered Homs through a dank storm drain with rebels from the Free Syrian Army. Above them, they could hear shells landing. Ahead, all was darkness. “Nobody told us it was 3 kilometres long,” exclaims Conroy. “It just kept getting deeper and deeper. You’re like, ‘Fucking hell, how long does this go on for?’ It was not a very comfortable feeling.”

Worse was to come. The city was under siege and civilians in Baba Amr were trapped with nowhere to run from the artillery and sniper fire that made “every journey heartstopping”. “The amount of weaponry deployed against them was just staggering,” says Conroy. “It really was beyond comprehension. Marie had been in Grozny, and she said she’d seen nothing like it.”

After a few days, word came that Assad’s forces were about to invade. Reluctantly, Conroy and Colvin left. When the warnings turned out to be false, “It was just the most sick feeling,” says Conroy. Their editors told them that they needn’t go back in; they could cover the story as people were coming out of Homs. “In our mind that would never have been good enough, because we weren’t there to witness it.”

Nonetheless, he told Colvin that he had a bad feeling about returning, which she dismissed, telling him: “I’m the journalist, you’re the photographer. I’m going in.” He knew how “stubborn” and “determined” she was, and didn’t argue. Had she listened to him, we wouldn’t be talking. Conroy discovered last year that a few minutes after leaving their safe house, a shell landed in the garden, igniting 750 kilos of TNT. “That building was vaporised. Nothing, for a quarter of a mile, was left standing. So if she had said okay and stayed, we’d all have been killed. When I heard that it was like, ‘Wow. That’s a bit of a mind-bender’.”

Back in Homs the story that would seal their fate came in the shape of a dying baby, whose final desperate breaths seemed to sum up the helplessness of the people of Baba Amr and the ruthless barbarism of the Assad regime. Conroy recalls the iconic image of the Syrian toddler Alan Kurdi washed up on Bodram beach, and how it had trained people’s minds on the refugee crisis. “For us the baby was that. It symbolised everything.”

The clock was ticking. The shelling was now unrelenting and Conroy knew as a former soldier that the end game was approaching. He told Colvin there was no time to file a story. “Most of the people we were in there with were dead. The bombardments had reached a crescendo that was unimaginable. There were people dying left, right and centre. The chances of us living long enough to get that story written and the photographs uploaded were nil. And we knew it.”

Instead, Colvin gave a number of heartbreaking interviews via Skype, finishing with CNN’s Anderson Cooper, in the full knowledge that this would draw more fire down upon the media centre.

“The rebels and activists were all listening, and we all knew that it was a bit of a historic moment,” says Conroy. “Or a moment when everything we’d all been trying to say about what was happening in Baba Amr came together.”

Up until then they’d only had “flimsy” conversations about how they were going to get out, says Conroy. “Marie was going on about dressing up in burkas and hiding in the rubble. I said, ‘Okay, do you think I’d look good in a burka, Marie?’ That was the depth of our escape plan.”

Their final moments together are burnt into his memory. They’d got up early to visit the field hospital again, and were creeping along a corridor quietly so as not to wake some French journalists who had unexpectedly arrived. “We were giggling away like a pair of kids,” says Conroy. “She went, ‘Oh fuck, the snipers will be up. And the French.’ And they were her last words to me. That was the last time I heard her speak.

“In hindsight, it was as funny an occasion as anything. All of the dark side was lost in them few moments. You know, I just remember that really fondly. That silliness of it. Here we are, at the shit end of the world, still worrying about the French.”

The blast that killed Colvin left Conroy with a critical leg injury. Rebels took him to the field hospital where the gaping wound was cleaned out with iodine and a toothbrush, and then closed up using an office stapler. Eventually, he was evacuated in an operation that left 13 dead.

Where the documentary ends is not the end of the story. Conroy rode across Syria on a motorbike for three more days, “shelled and hunted”. When he made it to Lebanon he stayed at the British ambassador’s house until he could be flown out, because Syrian death squads were searching for him in the hospitals.

A wrongful-death lawsuit filed in America against Assad in 2016 by Colvin’s family claims that the veteran journalist was herself targeted for killing. A judgement is still pending but Conroy is confident that they will win. The evidence is damning, he says, and will be a “game changer” when it’s all released.

“We have vast amounts proving this down to what they were rewarded, what cars they were given for killing Marie and getting me [a million dollar dead-or-alive bounty was placed on his head]. It was systematic. There was a whole military unit set up to track and kill journalists – we have that evidence.”

He hopes that in pursuing justice for Colvin, the case will also refocus attention on Assad’s other war crimes and “make it that slightly bit harder for him to be gently rehabilitated back into the diplomatic fold”.

As for Under the Wire: “There’s a lot of shit in the press, as we all know, about fake news. And it really irritates the life out of me that we’re all tarred and feathered with the same weird, broad Trump brush. So it is, hopefully, a little point in the right direction that there are a lot of people out there who risk a lot to tell people’s stories and get real news out,” Conroy says.

When I suggest that it is also a powerful challenge to Western pro-Assad propagandists such as Vanessa Beeley, whose conspiracy theories about the Syrian Civil War have spread like wildfire on social media, he cackles maniacally.

“I hope when people see this it will be quite hard to put this down to an imperialist CIA plot,” he says sneeringly. “That we weren’t there as part of a CIA mission – which I’ve read is the case. That I’m not an MI6 officer in charge of the west’s Syria campaign – which I’ve also been accused of.”

Would he like to debate someone like Beeley?

“I’m hoping to God she comes along to one of my Q&As after the film and I get a chance to go mano-a-mano with her, in public ... and, you know, just absolutely crucify her.”

Colvin and the long-suffering people of Syria deserve nothing less than the truth.