MY autobiographical film, The Closer We Get, documents the aftermath of two traumas in my family’s life: the discovery that my father Ian had concealed a half-brother, Campbell, from us for five years; and the devastating stroke suffered by my mother, Ann, some 15 years later, in 2008.

Although my film includes family archive stretching back decades, most of it was shot in the last two years of Mum’s life (she died in 2013) when my dad – separated from her for over a decade by then – had moved back into her home, and we had finally all got round to talking to each other, at last. It’s a poignant and surprisingly funny film, that seems to profoundly affect everyone who sees it.

I’ve known my half-brother Campbell (now 27) for 22 years, and I’m sure he doesn’t even remember not knowing me or my three siblings. Dad had met Campbell's mother, Tadalech, during a decade spent working in Djibouti in East Africa. They'd had an episodic but longstanding relationship. He was five years old when Mum made the very painful discovery of his existence. But from that point on, this endearing little boy became a fixture in our lives, visiting every summer and moving permanently to Scotland in his mid teens to study.

Despite my parents’ formal separation a few years later, my mother’s stoic sense of familial duty – and certainty that Dad could not function as a single parent – saw them share Campbell’s care and continue to see one another all the time, in our small hometown of Largs.

It was a peculiar separation of sorts, and one that made little impact at all on their growing brood of grandchildren, who’d never known anything different of their Papa and Nonna. So to interpret Campbell, or his origins, as a family "secret" feels strange to me. We’ve never felt ashamed of Campbell, who’d no part in what came before him. My mum – herself adopted as a baby – shared this sense, although in the film she confesses her deeper struggle to bolster her sense of duty with real maternal love, which remained elusive despite her best efforts. She was brave and proud outwardly, and she never denied who Campbell was to anyone, and as far as I know, she overheard no nastiness from friends or neighbours about him. That middle-class, small-town Scottish decency I so derided in my teenage years, I can now see was also a kindly, benevolent force.

Perhaps my mum knew she’d not live to see the finished film we worked on so closely together – she died during the long months of its editing, and despite my loss it was a great solace to continue our work, to laugh with her and share her wisdom on screen. By her last year, I don’t think she cared much for the future or the past, and this gave her an incredible serenity and levity that everyone who encountered her felt enriched by. What mattered to her latterly was what could be said or done right now, and there’s no doubt in my mind that our filming encouraged her to speak freely of feelings old and new, and somehow, to exorcise her pain by doing so. As a deeply altruistic person, Mum also, I think, believed that if sharing her life’s experiences on film could help others to deal with their own personal challenges, then it was her duty to do so.

Every other week I filmed at home with my Mum (still mentally sharp as a knife despite being almost quadriplegic) and her daily circle of care-givers – both professional and family (chiefly myself, my brother Mark and my Dad Ian). Each week I took away the footage – sometimes hours of it and sometimes just a few minutes. Stored it on a drive, watched it later. And always, I would find it full of miniature epiphanies, many of them joyful – a line of crystalline humour from my Mum; a pathos-filled silence following an exchange between Mum and Dad that felt like banter at the time. And from within these recorded moments, I began to tell the family story.

It’s this apparent domestic mundanity that my mother spent most of her life within, as a mother and home-maker, while Dad brought home the bacon as a respectable accountant – albeit as it turned out, a restless one. So it should be no surprise that she was so good at biding her time there, so adept at gleaning and making meaning there, amidst the drone of the dishwasher and daytime TV. I however, had spent my adult life avoiding this world, so to join my very sick mother and almost-retired Dad there, to kill time together trying to ignore her inexorable decline should have been a sad if worthy act of self-sacrifice. Instead it was revelatory, and not for the reasons you might expect.

When The Closer We Get was just a few lines in a notebook, in 2010 or so, I’d feel periodic waves of terror as I fast forwarded to sitting in a dark cinema surrounded by strangers gorging on the most profoundly sad period in my family's life. I imagined my dad vowing never to speak to me again, my siblings raging at their self-seeking exhibitionist of a sister who wouldn’t let bygones be bygones.

But that was before we had spent five years trying our best to become a team again, to bury our many differences and get on with fostering harmony and warmth around Mum for as long as she had left in this world. It wasn’t easy for anyone, but this sense of "love in action" felt vivid and special. It still does, and when something feels like that it I want to record it, which is what I was doing in The Closer We Get. Rather than dredging up and exposing secrets, confessions or confrontations, I was recording and sharing something revelatory: a family that was healing.

The occasional night terrors never threatened to stop my filming, mainly because Mum and I had actually begun planning this film before her stroke – in fact, we’d even filmed the unforgettable "life so far" interview seen in the film just three weeks before the stroke changed everything forever. The years since then did little to dampen her enthusiasm for the project – initially conceived as a two-woman investigation of Dad’s unknown-to-us life in Africa. So when we resumed filming, now with Dad well and truly within our – and the camera’s – reach, I felt as if I had a co-director in her, someone who needed this film as much as I needed it. In one bedtime scene, Mum and I speak of how the film will finally reveal to Ian many home truths he’s long ignored. She says to me starkly and not without mischief: "Well, so be it.” Only she knew for sure just how tough my Dad is, how he wouldn’t shirk from his "just desserts" – and also maybe she at last believed that he would never abandon us.

My father’s attitude to the film was always harder to fathom, in line with most of his attitudes then and now. Don’t get me wrong, like most of us mouthy Guthries, he’s an opinionated fellow. But when it comes to feelings, those were not and are not discussed. I learned young to read them and express them in other ways, as did all my siblings. It’s probably how and why I became a film-maker and artist. We’re great ones for sulking, door-slamming and sarcastic jibes. A recurring childhood memory is my Dad cajoling me with irritating efficiency into breaking a smile, when I was desperately trying to maintain a hard-won and lengthy sulk. There has always been a lot of laughter in our family, and there is a lot in The Closer We Get.

Nevertheless, I’d feel a stomach-churning terror when I’d periodically announce "a big interview" between us was scheduled for the afternoon – I felt like a small child again, confessing to having lost my school satchel. Yet Dad would plod upstairs dutifully after me, the camera would be set up and I would take a deep breath and ask a question I’d never dared ask before. These ranged from, "Were you faithful to Mum before Tadalech came along?" to "Did you miss us?" to: "What took you so long to tell us about Campbell?"

That these had never been asked before should tell you all you need to know about the family dynamic in 1993, when Mum called me (then a young art student) to say she’d discovered that Campbell existed. No-one could stand up to Dad. Despite the evident fact that our world was now officially upside down, the authority of the man – the distant but absolute monarch of our household – meant that he could and did do just as he pleased. And back then, that meant not explaining and definitely not saying sorry. Unthinkable as a life that encompassed our new half-brother was, it was just as unthinkable to imagine our picture-perfect family without Dad at the helm.

So, the answers to these simple questions were – to me, 20 years long-buried – the deepest secrets of our family. I didn’t want or expect full disclosure from a man as accomplished at evasion as Dad, but I knew that whatever I captured on film, and especially what was not said, would be enough. And something in Dad also clearly wanted to go on record. Maybe it was a way to tell me that he loved me and that he trusted me? These interviews are used very sparingly in the film and yet they are cornerstones of the whole experience of filming my family – rare and sincere attempts to connect.

I’ve made three documentary features previously (co-directed with Nina Pope) and none has been plain sailing when it comes to questions of privacy and ethics around our protagonists. We’ve always agonised over including difficult scenes or deeply personal confessions, and I have always thought that the dreadful pain of the first watch-through with one’s "cast" is the price you pay for the privilege of documenting a real human life in any meaningful depth: if you’ve done your work well, the chances are your protagonist recognises the honesty of what has been captured. It may be uncomfortable, but invariably we have retained good relationships with those we’ve filmed.

None of my family ever asked to see any of the film-in-progress as I worked away on it, although had I offered I’m sure they would have steeled themselves to try and offer constructive feedback. But I knew the film had to speak with my voice and that the struggle to find that was mine alone. I think the irrefutable fact that Mum trusted me so deeply, permeated my entire family’s attitude to the filming, and this gave me all the confidence I needed to tell our story.

I’m often asked how the film has changed me, to which I reply: "Not much. It was the experience I was filming that changed me.” My family still don’t talk on the phone every day, but we feel much closer than before. I miss them more now than I ever did as a child, or in the 20 years of adult life preceding my return home for the long farewell documented in The Closer We Get. Sharing a bereavement is a big part of that, naturally, but training my camera’s eye on those years of pain and joy showed me so much more than my own eyes ever could have, and I am profoundly grateful for that. I think my whole family is, too.

The Closer We Get is out now: thecloserweget.com