KAREN and Ann Guthrie had a grand idea for a film. Think Thelma And Louise, but mother and daughter, and set largely in their home town of Largs.

“We would make this probably quite funny, documentary/road movie where we went around tracking all my father’s secret places, where he lived and worked, meeting his friends, all the people he had worked with that we’d never met, and having this kind of adventure,” says Karen.

And then the sky fell in. Ann had a stroke, leaving her disabled. It was years before writer-director Karen picked up the camera again, and the results can be seen in an astonishing new documentary, The Closer We Get, out next week. Part portrait of caring, part mystery, the film is proof, were it needed, that there is no such thing as an ordinary family.

Life changed for everyone after the stroke. From visiting the family home in Largs once a year, Guthrie, 45, began commuting from her home in the Lake District up to Largs, then back and forth to London for her work as an artist.

“As time went on I realised I would have to give up more to be part of her life and to look after her. The film came back into my mind because I was thinking 'I’m in this situation, it’s not going to go away'. Maybe intuitively, instinctively, the part of my brain that was trying to cope with it said, 'Well, this is a way you cope with things, you’ve always made work, you’ve always been creative, it’s what you do, it’s how you get through life. This is going to be your salvation, your way out of this trauma.'”

Ann Guthrie, who had been a midwife in Glasgow before giving up work to care for Karen and her three siblings, did not hesitate. “She just said, ‘Yeah, let’s get on with it’.”

The camera was left to run for long stretches to catch the everyday rhythms and details of family life. Time passed, nothing seemed to happen, then mum would come out with “one of her fantastic bon mots” that made the scene. It was the opposite of the cut and thrust, pacy style of documentary making. “We never do that as filmmakers. We don’t do it as people, just sit, not saying anything and waiting for that bit of repartee.”

The protagonists in this picture were not just Guthrie and her mother. Though dad Ian was divorced from Ann, he moved back into the house to help care for her. Also present was one of Karen’s brothers, Mark. Did they ever get fed up with the camera and say "enough”? Mark probably used to dread walking into the living room and finding the camera, she laughs. “But with mum and dad we genuinely all used to forget the camera was there.”

The Closer We Get is about more than what happened to the family after Mrs Guthrie’s stroke. There was a reason why mother and daughter wanted to make that original film. For a large part of the children’s lives, some 10 years in total, Mr Guthrie, an accountant, had worked in Africa. At home in Largs he was dad; but who was he thousands of miles away?

Guthrie is as meticulous in pursuit of answers to this question as she is in charting her mother’s physical decline. Most families instinctively keep their secrets close. It cannot have been easy, I venture, to be so open. Guthrie acknowledges that the film is very much her interpretation of events and her siblings may feel differently.

“I think my parents are both treated as real people in the film. Mum is not a vulnerable, disabled woman that you should just pity. Dad is not a man you should point a finger at and paint in a black and white way. I hope that both of them are shown in a nuanced way. But it is my story so I can’t pretend that my siblings would agree with everything that is in it.”

Helping her keep some distance was the film’s editor, Alice Powell, and Guthrie’s long-time producing partner, Nina Pope. “It’s all about what you leave out," says Guthrie. "Every filmmaker will say that to you.”

Mrs Guthrie died in 2013. Though this is the story of one family, Guthrie hopes it will chime with anyone now getting to the stage in life where they are caring for a parent. Guthrie looks back on the experience as “fantastic”.

“It might sound peculiar but although it was the most tragic time in my life it was also the best of times. To know that every other week I was going to be in that bubble for a few days. The rule book was thrown out.”

Mrs Guthrie had personal carers coming in, allowing mother and daughter the time, space and energy to enjoy the time together, reading, chatting, putting on face packs. It was a different woman she got to know.

“Because we had had quite a bristly relationship when I was younger I think I found a way to shut the door on that and say, 'Well, I’m fascinated by this new person. Yes, she is still in there but there is also this new honesty, this new serenity, new wit, all these things that were sort of there in the beginning but had been ground down.' I thought, 'I like this new person, I love this new person, I’m going to spend some time with her.'”

The Closer We Get had its premiere at the Edinburgh International Film Festival in June, and a proud dad came along as part of the Guthrie clan. Together, they have come out the other side of an experience that awaits many other families.

“We’ll all go through it,” says Guthrie. “If we look at the next 50 years of life, not just in Scotland but in the UK, we are all going to have to negotiate this stuff. We all need to find different ways to live as family units as honestly as we can do it, but also we need humanity. We need to be able to forgive each other for things we’ve held against each other for a long time.”

Since making the film, Guthrie has been working with organisations including Chest, Heart and Stroke Scotland.

“So far we are at the stage of editing a suite of clips from the unseen footage of the film, specifically for the training that CHSS do with health professionals and NHS staff, to help them understand how care for stroke survivors pans out in the home.”

In an email from Japan, where she is working on a new art project, House Of Ferment, which she describes as “a kind of self-portrait in kitchenalia and food”, Guthrie says her mother, who did a lot for charity, would approve. “It’s a fitting memorial to her to do all that we can to help others with this precious material she so generously gave us when alive.”

[itals] Vikingar, Largs, September 18; Dunoon Film Festival, September 19; Glasgow Film Theatre, September 20. Other screenings: see www.facebook.com/TheCloserWeGet