As the script was running short, McCallum made a suggestion in a scene where the enhanced Gwyllm is flipping through books and then proceeds to play complex Bach pieces. “I remember the line was ‘Playing the piano is a simple matter of mathematics and manual dexterity’!”

In fact, McCallum might have pursued a career in music. “My father and mother were both musicians. I became an oboe player and my father offered to send me to Paris, to the Conservatoire, and study the oboe. By that time I was hooked on the stage and said, ‘No, I’m going to go on the boards.’ And I did.”

He has returned to science-fiction quite a bit over the years, fronting the US series The Invisible Man, co-starring in ITV’s Sapphire and Steel (more of which later) and appearing in episodes of Babylon 5 and SeaQuest. So does he like it as a genre? “I’m more of a practical man,” McCallum replies. “I was very happy to get to work and get paid for it, pay school fees and insurance, you know. This is the life of an actor, particularly in the early years before you get a little better known: your main concern is to make enough money to keep the family running.

“Fortunately I married a young lady who was more talented than I was [his second wife, model and actor turned interior designer Katherine Carpenter] and so we shared the burden together and we’ve had a very successful relationship. In fact, we’ve been married for 55 years.”

But back to 1963, a year that ended up being a busy one for McCallum, with the release of the classic war film The Great Escape, in which he played “dispersal” expert Eric Ashley-Pitt. Does he have any outstanding memories? “Yes, the Odeon Leicester Square… the front rows of the upper circle all reserved for cast and crew. We sat there, that enormous red curtain parted, with Elmer Bernstein’s music playing… it’s a moment I can remember as if it was ten minutes ago. I mean, imagine you’re in a film like that and you’re seeing it, and everybody else is seeing it, for the first time. It was quite amazing.”

McCallum couldn’t quite escape PoW camps – he went back inside for the BBC series Colditz, which began 50 years ago. “Again it was a wonderful thing to play the part and he was a very interesting guy, the man that I played [RAF officer Simon Carter, based on real Colditz inmate Flight Lieutenant Dominic Bruce]. Pat Reid wrote the book on which the series was based – it was all very gung-ho and hail-fellow-well-met…

“There’s a new book that’s come out [The Prisoners of the Castle]. It’s a detailed study of Colditz and tells you all the terrible things that were going on between the individuals who were actually incarcerated and everything that the Germans and the war effort were doing at that time. It was an eye-opener to read that.

“But back then, when I went to visit Colditz, it was in East Germany, so that was quite interesting and dramatic.”

Speaking of the Cold War, McCallum was already a huge star in the ’70s, thanks to the spy series The Man from UNCLE (1964–8), playing Illya Kuryakin, a Russian agent to Robert Vaughn’s American Napoleon Solo. So what quantities of fan mail are we talking? “Oh, huge sacks of it. Obviously we couldn’t possibly deal with it ourselves, there was a whole department back then.

“It was before social media and silicon chips, so it was practical matter of opening envelopes… the studio took care of it but I believe they got more fan mail than most.”

And presumably, he was recognised everywhere after that? “Yes, oh yes, I totally lost my privacy.”

Illya and Napoleon were very different characters who blended incredibly well. Was that the case off-screen with Vaughn? “We worked together and we went to our separate lives and that was it. I think I went out to dinner once with Robert. He invited me because he was a health food nut and used to go to a very early health food restaurant and I was dreadfully sick the next day. I thought, ‘Health food is not for me, I’m going to eat my bacon and eggs.’ I’m in my 90^th year so whatever I’ve been eating seems to have worked!”

Another great TV pairing was with Joanna Lumley in the supernatural series Sapphire and Steel, which ran from 1979 to 1982. Was that an enjoyable experience?

“Enormously, because I got to meet Jo and Jo’s now a good friend. But what is very interesting in that relationship – six degrees of separation and all that – is that Joanna has a son and Katherine and I have a son, Peter, among other children… But there is a set of twins on this Earth, two ladies, and James in London, Joanna’s son, was dating one of the twins and Peter here in New York was dating the other one. Now if that’s not a coincidence! So I think Sapphire and Steel had a life of its own and that’s one of the little comments that was coming down from an ESP place!”

Sapphire and Steel is a chilling series that many enjoyed without fully understanding it. Was that the same for McCallum? “It was in the beginning but then I talked to Peter [Hammond] who was writing it and Shaun [O’Riordan] who directed it and I said, ‘My mother’s cleaning lady, Mrs Puttock, used to say she loved watching Sapphire and Steel but she didn’t understand a word of it. And I said the scripts from now on have to be made Puttock-proof. It’s terribly important that she understands it.’

“In the family vernacular whenever I get a script that’s confusing, I say [to the writer], ‘You’ve got to make this Puttock-proof.’ And then of course I have to explain about my mother’s cleaning lady!”

ITV

But perhaps McCallum’s most successful series is the one he’s been in for 20 years: NCIS. “I have two shows left on this contract. I’ll be going out [to shoot it in California], health permitting, if and when they need me. I have a feeling it’s going to be the last two shows of the season. Usually they like to have everybody in those.”

And yet McCallum has mixed emotions about the work… “Travelling is not quite as easy but more importantly than that, when I get there I know very few people because the show of NCIS that I did with Mark Harmon [Leroy Jethro Gibbs] and Cote de Pablo [Ziva David] and a whole bunch of them, they’ve all moved on to other things. So when I go there it’s like being in a different show [laughs] and it doesn’t quite make sense any more.

“But I have a wonderful relationship with my assistant Jimmy Palmer. Brian Dietzen [who plays Jimmy] is a wonderful actor. He’s just been a rock, a tremendous guy to work with.”

CBS Photo Archive/Getty Images

So is retirement not a word in his vocabulary? “Well Katherine retired many years ago but she’s still working. We both… retirement is a work in progress, shall we say!”

The Outer Limits: The Complete Original Series Blu-ray Box Set will be released on 12th December and is available to pre-order now.

All 34 episode of Sapphire and Steel are available to stream on BritBox – you can sign up for a 7-day free trial here.

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