Brian Blessed is the quietest, stillest, man I have ever met.” Thus spoke Kenneth Branagh recently when asked to describe the man he cast in four of his Shakespeare films, and who returns to the stage this week in Peter and the Wolf.

Come off it Ken! From where I'm sitting, the actor/explorer/national treasure is as boomingly buoyant as the figure of lore. His 5ft 9in frame zipped into a red fleece, his eyebrows and beard tangled like jungle creepers, Blessed's biblical tones bounce off the oak panels of the rural Berkshire hotel he has chosen for our interview. Down the road is the house he shares with his wife, the actress Hildegarde Neil, and “numerous dogs, ponies, cats, ducks and fish”.

He is just back from Borneo, where he scaled the region's highest mountain. It turns out that The Knowledge's photographer once tackled the same peak. “Have you?” he roars, fixing him with a fiery stare. “Bastard, isn't it?”

You don't interview Blessed so much as witness him. Changing the course of one of his anecdotes is like a tug boat heaving at an oil tanker. He is an incorrigible name dropper, managing, in one memorable five-minute burst, to squeeze in mentions of Muhammad Ali, Pablo Picasso, Katharine Hepburn, Tom Cruise and Stephen Hawking. “That sounds a bit big-headed,” he says after one bombastic recollection. Perhaps. But how could a man of such mighty appetite, ambition and vocal cords not have an ego to match?

He wastes no time in describing his fitness regime. “I run seven miles a day and I do two-and-a-half hours of weights, and I'm 71.” Intrigued by Blessed's expeditions to Everest (he's attempted to climb the mountain three times, but never reached the summit), Russian scientists tested him with a view to sending him into space. They told him he had the physique and constitution of a man 30 years younger. He doesn't seem surprised. The project is on, and he has already had his “centrifuge” and “plummet” training.

But before that, Blessed has a terrestrial appointment as the narrator of a touring theatrical production of Peter and the Wolf. Anne Geenen's story includes a new prologue, which introduces Prokofiev's characters and contains an ecological subtext that appealed to the current president of the Council for National Parks. “We all need to get into the wilderness,” he exhorts. “We're thriving cripples: in our cities, our hearing, our eyesight diminishes. After four weeks in the wilderness, you start to see and hear better.”

He has an affinity with both Peter - “I went into the woods, the deepest woods, and I love danger” - and the Wolf, having recently spent 20 days horse-riding in Mongolia, where he and his guides “were followed on both sides by hundreds of wolves”. He went on to solo climb a local mountain “with a big dog that was half wolf. It climbed alongside me. I fed it and it slept in my tent.”

So which takes precedence: acting or exploration? “Acting's a must, good, bad or indifferent. But my biggest love in life, since I was a child, is exploration. I love being on my own in the wilderness.” That, he insists, is where the quietness and stillness that Branagh speaks of come to the fore.

Blessed was raised in a Yorkshire coal-mining village called Goldthorpe, halfway between Doncaster and Barnsley. “Running alongside us was the North Eastern Railway,” he remembers. “The Mallard! The Flying Scotsman! I almost fainted with shock! They're not trains today, they're little buses.” As a boy, he would wait for his father, William, to come home from the pit. “To see him walk through the mist with his helmet shining and his black face, he looked like Hector at the gates of Troy.”

Like many young Yorkshiremen, Blessed spoke his mind. When he was 11, his father took him to the World Peace Congress in Sheffield, where the guests included Picasso. “I said, ‘You're not Picasso, you sound more like Carmen Miranda. Prove it - draw me something.' He drew the famous peace dove, but I said, ‘That's not a dove!' and gave it back to him. He looked around and said, ‘It's the first time I have a true critic.' The drawing is now in the Sheffield City Hall and worth £11.5million.”

Patting my knee as he recounts his Zelig-style tales, he is a strange mix of warrior and luvvie. Despite all the Boy's Own sadventures, he insists that his real heroes were women: his mother, who used to break up fights between miners, and Katharine Hepburn, with whom he starred in a 1971 film called The Trojan Women. “She was 60 and I was 32. I was deeply fond of her. We were filming for 15 weeks - another five and one would have been proposing to her.”

After training at the Bristol Old Vic, he had made his acting mark in Z Cars (1962-65). Since then he has played a string of muscular patriarchs: Augustus in I Claudius (1976), the hero's father in Robin Hood: Prince of Thieves (1991), the dual role of Duke Senior and Duke Frederick in Branagh's As You Like it (2006). He doesn't get annoyed about being typecast: “I don't get annoyed about anything.”

The armour is impregnable. Was there ever a tension between the gruff machismo of his father's coal-mining world and the flamboyance of acting? Never, he asserts, citing the infamous Old Vic production of Macbeth in which he played Banquo opposite a critically eviscerated Peter O'Toole in 1980. Near the end of the run, Blessed found O'Toole in his dressing room, “on his knees, screaming like a banshee, destroyed in heart and soul. I said, ‘F*** it Peter!' - the audience were only coming to see him suffer. But he said, ‘No - I'll go on.' He completed it and completed the run. I have never - never! - seen courage like that. My dad, who once saved 300 lives down the pit, said he knew of no one who had the courage O'Toole displayed that evening.”

The voice has swell-ed during that bid for entry in Private Eye, and people are looking our way. Does he mind being seen as a larger-than-life character? “I love it. I suppose I am.”

He has certainly embraced the cult status conferred upon him by his role as Voltan in the 1980 film Flash Gordon. He recalls a sci-fi conference in Chicago: “They would fill the auditorium, I would go in and say, ‘Gordon's alive!' three times and they just screamed and took photos. Then they would fill the auditorium again. I did that for seven hours.”

He won't entertain the idea of slowing down: “I'll always be active. My dad died at 99 - he was very active until he went.” The trips will continue, he says. “You don't go up Everest to die, you go up Everest to live.”

His wife, apparently, agrees on his indestructibility. Next on his list is Sangay, a volcano in Ecuador. After that, he would quite like to descend to the bottom of the Mariana Trench and balloon around the world with Richard Templeman-Adams. Then, of course, there is the final frontier. “See you in space,” I call as he bustles off. He turns and booms over his shoulder: “I'll wave!”

Peter and the Wolf is on nationwide tour until April 20

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