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That milestone is celebrated in the 72-year-old's new one-man show, Being An Actor: 50 Years On, filmed at the Traverse last weekend and due to be streamed globally by Shedinburgh at 7.30pm, this Saturday, August 28.Acclaimed as a stage and screen actor, writer and director, Callow is also well known for his performance in the National Theatre’s production of Amadeus, in which he played the title role, and big screen appearances in films such as A Room With A View and Shakespeare in Love.
Today he is enjoying a coffee in Stockbridge when we meet. He's back in Edinburgh, a city he adores and where it all started for him, and it's clear he couldn't be happier to be back, although he has a confession to make - it's not quite 50 years.
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"It's actually only 47 but it certainly feels like 50 years," he laughs, "As you get older you think in decades, rather than years. When you're young you think in months, when you're very young you think in terms of days, but as you get older it just races along. You feel as though you are sitting on a giant's shoulders and the giant is called Time and he is striding forcefully through life."
Being An Actor: 50 Years On finds the effortlessly charming Callow wondering, 'What is acting?' Inspired by a book he penned having been in the profession for just a decade, he is very aware of the impetuousness of youth.
He says, “In 1982 I wrote Being An Actor, it was my reckless attempt, after not even ten years of acting, to describe the physical, psychological and emotional experience of creating and playing characters. The cheek of it. Now, after nearly 50 years of immersion in it, I am still obsessed with trying to plumb its mysteries, and will try, before your very eyes, to catch at its essence.
"My professional life started in Edinburgh in 1973 so, of course, it is in Edinburgh that I will present Being an Actor: Fifty Years On for the very first time.”Callow made that professional debut in that year’s International Festival production of The Three Estates, at the Assembly Hall on the Mound.
"It was very exciting because it was The Three Estates, which was a big deal because it was the first time it had been revived since the death of Sir Tyrone Guthrie who had rediscovered it and made it such a huge success in early Edinburgh Festivals," he recalls.
"Every famous Scottish actor of the day was in it, Roddy McMillan, Tom Fleming, Lennox Milne, Johnny Grieve, Wally Carr... as a young actor I loved it.
"It was my first exposure en masse to Scottish acting and it was a very definite thing, the profession here was very much more tight knit than the one in London and all the Scottish actors, in those days, were equally adept at variety, Johnny Grieve was a great comedian."
Remembering arriving at the Waverley Station that first time and waking up at the Lyceum where he was rehearsing, he says, "I felt on my way to something extraordinary."
Gazing across to the Water of Leith across from where we're talking, he adds, "At the moment I'm staying in Stockbridge, which I love. It's my favourite place to stay, but when I came up for that first Festival, nobody had bothered to book me a place to stay. So I went around the city and eventually found a place in Broughton Place. It was a pretty dismal, with a bed that folded out even though the room was too narrow for it to do so completely - my feet were in the air as were my shoulders. The window was smashed and there were mad people running about all over the place – but I just loved every single thing about it.”
After that Festival, Callow headed to Lincoln where he appeared in Rep for four months before being tempted back to the Capital.
"I was invited back to Scotland to do the great Edward Bond play The Narrow Road to the Deep North at The Netherbow and the musical The Fantastics, which took me all around Scotland, that was great. I got to see Scotland and discovered it was in my very fibre, my heart, in my bowels.
"So I have always had a love for Scotland and Edinburgh; I directed My Fair Lady at the Playhouse and Carmen Jones at the Festival Theatre and I've acted at The King's and know Edinburgh very well."
Looking ahead to Being An Actor: 50 Years On he reflects on the difference the intervening years, since writing the book in 1982, have made to his analysis of the art form.
"It becomes more complicated as you go on in your profession. If you love it you keep finding more and more extraordinary things about it, more and more possibilities, more and more challenges and more and more difficulties.
"People are rather fascinated by acting it seems to me, mostly because we are all acting all the time, the only difference is that some of us are being paid for it.
"Everyone is putting on masks all day long, adjusting their personality to a situation, maybe not as extremely as I would if I were going to play an 18th century mass murderer perhaps - I would take it a little bit further than most people I hope – but I thought it might be nice to take an audience through, not just the phases of an actor's career, but also the phases of his work: you start in the primeval swamp of unemployment then you get your job. You rehearse it and then you do it. Then you have a run and then you're back to the primeval swamp again.
"That's basically the cycle, but then I want to try to get into what it is you are really trying to do as an actor. The answer of course is immensely complex and varied."
Despite his achievements and recognition as a national treasure, Callow admits to remaining as insecure as any other actor in his profession.
"As I sit here, I'd like to quote the great Irish actor Cyril Cusack who said, 'Every job you do feels like the first job you have ever done and every job that comes to an end feels like the last job you will ever get.'
"There is a constant insecurity in the life of an actor because you are a commodity dependent on the favour of the public and people can get tired of actors. I understand that very well. That's why I've tried to come out of a lot of different traps, directing plays and films and writing, which has taken me away from the acting treadmill, although it might have been better if I had just dedicated myself to it, I don't know.
His personal highlights of being on that 'treadmill' include Amadeus, Faust at the Lyric Hammersmith, and the film A Room With A View.
"Four Weddings and a Funeral was a great thing to do as was directing Alan Bennet's play, Single Spies, in the West End and a one-man show I did here at the Festival called Tuesday at Tesco’s, playing a transvestite, those were all incredible, thrilling moments were you achieve a certain contact with an audience and have a feeling of hitting the nail on the head," he says.
All of which will no doubt get a mention in Being an Actor: Fifty Years On.
He beams, "It's the first time I've talked about these things at any length and I'm so happy it is in Edinburgh that I'm doing it. Eventually, I hope it will be a very full and elaborate production but this will be my first attempt at wrestling with the octopus of acting."
The Shedinburgh performance will be streamed Saturday, August 28, 7.30pm, tickets available on a Pay What You Can system starting at just £6, www.shedinburgh.com