THERE can't be many times when an actor finishes a film and proclaims she'll be very lucky to do anything so exciting again before she dies. But this is exactly what Sheila Hancock says of the time spent last summer in Sutherland, shooting her latest feature, Edie, climbing the camel-humped peaks of Suilven, sleeping under canvas and squelching through Highland bog. “It was glorious,” the 84-year-old recalls. “I can never match it, quite honestly. We climbed that bloody mountain. The crew climbed it with cameras and sound equipment. We all felt profoundly involved in it, in a way that I never have in any other job.”

This is quite a statement from Hancock. After all, she appears to be no stranger to new experiences stranger to new experiences; someone who throws herself at life all the more now she is in her 80s, all the more since the death in 2002 of her husband, Inspector Morse star John Thaw. “One of the things that happens when you get to my age is you realise you’ve only got limited years," she says. "With the best will in the world, you can’t go on for ever. You are greedy for experience. I weep for the old people that choose to live in a bungalow because they think they can’t climb steps. If you give up you can’t, but if you don't you can.”

Just being in that remote part of Scotland, she recalls, blew her away. “I’m a city girl really but, my God, the landscape. Probably the most arduous thing was, there are no roads leading to this particular mountain so we had to go over squelchy bog, had to row across a loch, and it was amazing.”

Hancock, who grew up in London under a King’s Cross pub which was run by her mother and alcoholic father, was evacuated during the Blitz, and has lived most of her life in the city, had always liked Scotland. But she had never explored its mountains in this way before. She recalls how, at one point in the shoot, the gillie drew attention to a black patch in the distance. “He said that it had been there since the beginning of time. Geologists come from all over the world to look at this. You just can’t get your head round this.”

In a way the film is Hancock’s The Revenant. Leonardo DiCaprio described that movie as his toughest shoot ever and for Hancock, making Edie took her physically beyond the call of duty. In the months beforehand she trained hard, went to the gym every day and took herself off to do Nordic walking in bogs. During the shoot, there were moments she felt mild terror. For instance, when she had to walk a narrow pass, around one metre wide with sheer drops on either side. Or when, as a helicopter did a pass to take a shot, they were almost blown off the 731 metre-high summit. “All sorts of funny things like that happened," she recalls, "but God, it was exciting.”

While she was filming she would send photographs to her youngest grandchildren, who thought she was climbing Everest.

All this, in order to tell the story of 83-year-old Edie, the gruff and worn-down carer for a controlling husband, who, on his death, heads to the Highlands to attempt the mountain that, long ago, her father had wanted to climb with her.

On some levels you couldn’t get a woman more different from Edie than Hancock. The actor, who has appeared in television from EastEnders through to Casualty, and films from Carry On Cleo to The Boy In The Striped Pyjamas, has always seemed an unstoppable life force. She was like that even when, as she documents in her 2004 memoir, The Two Of Us, she was struggling to live without her big love, John Thaw; even when, as she relates in the follow-up, Just Me, she had to battle to embrace life on her own.

But Hancock does have some things in common with Edie. She is, she says, “acquainted with death”. A widow twice over, she lost both husbands John Thaw and Alec Ross to oesophageal cancer. “I have cared for two sick husbands and a mother,” says Hancock, “and I know the utter confinement that can bring, where your life is on hold. I only did it for a relatively short time. With my first husband it was about a year, and with my mum it was several months. John was always very independent – but nevertheless I had to be there.”

She knows too that loss can also bring freedoms. In Just Me, she wrote about her discovery of travelling alone: “I had gone from one husband to another husband and I’d always had a male with me to do things and be with and be company and all that sort of things. It seemed to me that one of the biggest challenges was doing not only new things but old things, on my own. So that was what I wrote the book about.”

Edie’s character was one she was instantly attracted to. Life-affirming roles like these, Hancock says, are few and far between. “When I get a part, my children always say, ‘Oh, do you die or do you go demented?’”

One of her favourite speeches in the film is delivered to Jonny – the young man (played by Sunset Song star Kevin Guthrie) – who helps Edie prepare her for the climb. I haven’t been happy at all, she tells him. Don’t let that happen to you. It’s this exhortation to go out there and live that she particularly loves. Did she think the role of Edie was in some way made for her?

“To be perfectly honest,” she says, “I think the only reason I got the part, which any actress would give their eye teeth for, was because I'm fit. There aren’t many 80-year-olds who could actually physically do it.”

Hancock, with her slender but muscular frame, looks like she could be a walker. But mountain-climbing hasn't featured much in her life till now. “I do walk a lot,” she says, “but just along the river.” Over the years, she's had to be fit for her performances in musicals such as the UK premiere of Grey Gardens last year and Cabaret in 2006. “But I wasn’t mountain-climbing fit. I knew I had to get into training.”

The running joke around the crew during filming was that Hancock was working through her senior Duke of Edinburgh Award. “Whatever new thing I did like cycling or rowing, I thought, ‘Well I’ve got that badge now.’” Her least favourite, perhaps, was her camping badge. “Why does anybody camp?” she exclaims. “It’s utterly dreadful. One of the camps was halfway up the mountain on a sort of ledge, and it was above the clouds and I have never been so cold in my life. I kept everything on. All our clothes plus thermals, plus two sleeping bags, plus a tent that was supposed to be the best tent in the world.”

That Hancock reached the top of Suilven even impressed the locals. “Most of them hadn’t done it themselves," she says. "There’s a point you can go to where you don’t actually go to the top and often they’d gone to there. But nobody my age had ever been to the top.”

Edie's profound reaction to the landscape comes across strongly in the film. It’s there, in Hancock’s eyes and across the shifting, make-up free, topographies of her face. Hancock was genuinely moved: “I’ve never in my life felt so immersed in nature as I did in that landscape. What’s wonderful is that I didn’t feel diminished. You’d think standing in a landscape like that you would think, 'Oh I’m a mere human being, tiny in this thing.' I didn’t, I felt part of something huge. I felt part of it.”

Hancock hasn’t actually seen the film; she never watches herself. It’s something she’s avoided since she was a young actress. She tells me she felt destroyed while watching the rushes of an early big role in Karel Reisz's Night Must Fall. That evening, Reisz took her out gave her some advice. “You’re one of those people who should never watch yourself because that scene was really good," he told her. "You’re just an appalling judge. So don’t do it. It’s destructive.”

The new film is one of the many things that she wishes Thaw was still around to see and share with her. “I wish he could see it. Because he was so generous and loving about my work – and critical. So therefore I did value if he thought I’d done something really well.”

Unlike Edie, Hancock, is not someone who hasn’t embraced things. She continues to throw energy at a huge range of causes, from her vice presidency of St Christopher’s Hospice in London to countless projects helping young people. She's always been like this – so much so that Thaw used to joke that she had a Messiah complex. The day before we talk, she has spent at a school in Liverpool where she helps out with an IT programme for children who are dropping behind, and visiting a building project which refurbishes derelict houses to be used as low-rental homes.

That project chimes with Hancock's wider feelings about housing and the working class. In her 1987 book, Ramblings Of An Actress, she wrote about the need to rebuild working-class communities, and her novel, Miss Carter’s War, also touches on the issue. “I think part of the problem,” she says, “is architectural. When I was looking at all these terraces yesterday, I thought – bloody hell this is the way to live.”

Having published, over the last decade, three books, including her first novel at age 80, she is now writing about Brexit and the current political situation. Memorably, in the run up to last year's EU referendum, she stole the show in a Channel 4 debate with her passionate defence of the EU. “I do find it devastating,” she says, “that we seem to be building walls in America and here, and rather than uniting the world, we seem to be going apart. The same is true of society. Divisions are growing. The work that I do is with people who are at the bottom of society and having a real struggle. And then you see the utter vulgarity at the wealthy end.”

The tragic Grenfell Tower fire has affected her terribly. “I drove past it yesterday coming back from Liverpool and I can’t bear it.”

At 84, Hancock seems at the top of her game, determined not to let expectations of old age get in her way. Too often, she says, that’s what happens. People lose their confidence. “I find myself doing it. I live part of the time in France and I had to drive from my house down to Antibes. I actually found myself thinking, ‘It’s a bit old to be driving on a French road.’ What am I talking about? My driving is as good as when I was young, if anything slightly better. But it’s very easy to cast yourself as an old person who can’t do things.”

Hancock clearly isn’t going to cast herself in that role. Among her plans is a return trip to the mountains of Scotland as soon as she can. “I can’t say I’ll climb or scramble, but I do want to go back to that landscape and walk. I wouldn’t live under the conditions I do in the film, but I do want to explore it, because it is breathtaking.”

Edie screens at the Edinburgh International Film Festival on Monday 26 and Wednesday 7 June,

The Sunday Herald is the festival's media partner