DAVID McCallum got his Equity membership as long ago as 1946, in the days when his voice was often heard on BBC radio, and he made his first appearances on television in 1953. His distinguished subsequent career includes such high-profile films and TV series as The Great Escape, The Man from U.N.C.L.E., Colditz and Sapphire & Steel. Currently, he can be seen in NCIS, one of the most-watched shows on American television.

All of which is a roundabout way of saying that he has been around for a long time (“don’t remind me”, he says), and now, in his ninth decade, he has produced his very first novel. A good read it is, too.

Once a Crooked Man revolves around an intermittently successful New York actor, Harry Murphy who, while relieving himself in a garbage-strewn alley outside a restaurant, hears three brothers – Sal, Max and Enzo Bruschetti, all of them wealthy, ageing gangster bosses – discussing their plans for retirement. Murphy, spellbound, furtively makes notes of the conversation; he quickly realises that their plans include terminating, with extreme prejudice, a man by the name of Villiers, who lives in London.

Unable to ignore what he has heard, Murphy tries and fails to reach Villiers by phone, so he does the decent thing: he flies to London and thus sets in train an entertaining, and occasionally dark series of pursuit-studded episodes. Partly by calling on skills he has learned in his trade, he becomes quite an adversary for the ruthless brothers. He also encounters a clandestine task force that works with MI5, MI6 and GCHQ, and an equally secretive coven of computer hackers.

It sounds as if the research was fun. Yes, McCallum says. “The research was very interesting because I got to work with the DEA [Drugs Enforcement Agency] and check up on various things, and I’ve been to various places where hackers all gather together.

“They have a thing called the Def Con conference – last year it was in Las Vegas – where all the hackers get together and share information. The government goes in and looks for the really smart ones and recruits them on counter-cyberterrorism. In general, yes, I had a great time doing it.

“I’ve been doing [the book] for a long time,” he adds. At first, it was just an exercise in learning how to write. “I sat down and started to write, and over the years, literally, it gathered together until maybe three years ago when I decided it was time to stop being foolish, and I sat down and finished it.

"I do a lot of flying [between Los Angeles and New York, where he lives], and that's four or five hours each way, so there was a point where there was a lot of pressure to get it edited and finished. I used these trips back and forth to home to do a lot of typing.

"The idea for the book came before we started doing NCIS, and we've been doing that for 13 years. It's probably about 15 years ago that I first started it, though it was in a drawer for months, possibly years, in the beginning."

Once a Crooked Man is published in this country by Dingwall-based publisher Sandstone Press, the 14-year-old imprint whose books have won or been shortlisted for everything from the Man Booker to the Arthur C Clarke awards. "They read it and liked it. It's quite extraordinary," he says. "I think the [general] reaction is, people will think, 'oh yeah, actor David McCallum, he's written a book, yeah, okay'... There's a great deal of scepticism... And then people read it, and for some extraordinary reason they seem to enjoy it and end up helping me to promote it."

I mention that it's something of a surprise to see Murphy becoming quite so resourceful in the face of danger. "Surprise is very important," his creator concedes, "but he's still very naive, though."

Is there anything of McCallum's early acting career in the chapters when we see Murphy auditioning for such things as a mayonnaise commercial? "The life of an actor in New York – or anywhere, for that matter – is a constant search for work," he observes. "I've seen people sitting at rehearsals with their diaries and address books, calling people to find out who's doing what, and where. They'll scan the trades [trade magazines] too.

"To a certain extent, even once you're established, unless someone needs you at a particular moment, you're out of work. If you have a family, it's a very, very tough life.

"I'm not quite sure what it's like out there today: it's a very different world out there. When I grew up, there were lots of repertory theatres I could work in, and film and television. It's not as easy life, but you deal with it."

McCallum, the son of two professional musicians, was born in Glasgow in 1933, and moved to America in 1961. Between 1964 and 1968 he played the enigmatic Russian agent Ilya Kuryakin opposite Robert Vaughn’s Napoleon Solo in The Man from U..N.C.L.E. (the acronym stands for the United Network Command for Law and Enforcement, a multi-national agency charged with “maintaining political and legal order anywhere in the world”). The role made him a heart-throb. At the height of the series’ popularity he made a personal appearance at the New York department store, Macy’s, to autograph copies of his latest album of self-arranged, largely instrumental music; such was the press of avid female fans that Macy’s had to shut down Herald Square, in front of its main entrance.

It’s been a good few years since he was back in his native Glasgow. “I remember it more when it was black and dirty,” he says. “I don’t remember it so much, all cleaned-up. I was there throughout my childhood then we left for London, and when the war broke out I was evacuated up to be with my aunt in Stirling.”

So far as his own taste in authors is concerned, he has a scientific mind and enjoys reading Richard Feynman, although “I’ve read Stephen Hawking but don’t understand it at all." He also likes books on politics and current affairs. Fiction? “As a child I read Sherlock Holmes and that progressed into the famous detective writers – they were on the shelves at home and I just pulled them down and read them.”

He’s happy still to be working, playing Dr Donald (“Ducky’) Mallard in NCIS. “I don’t know how much longer [he will be working], but we’ll see, we’ll see. Ducky’s a great character.”

His US publisher has asked him to write another novel. “I discussed the idea of doing a sequel to Once A Crooked Man, particularly bringing back a key female character, but they said nope, see if you can come up with a book that has Harry but not necessarily with the other characters. That’s what I’m working on.”

He plans to give some of the proceeds from his debut novel to the Marine Corps Scholarship Foundation, which helps sends children of Marines to college. Last year he and his wife Katherine were honoured with the Semper Fidelis Award at the Corps’ annual West Coast Campaign Celebratory Gala in California. “When I’m promoting the book,” he says, “I’m thinking more about sending things to college and not worrying too much about anything else.

“In having a second career, and maybe a little income from it, it’s very good to know that my grandchildren will be taken care of with their education.”

McCallum’s admiration for the Marines obviously runs deep, for he has affectionately dedicated his book to “a man I never met”: Lance Corporal George Whitney Carpenter, of the US Marine Corps, his wife’s late brother. Carpenter was just 21 when he was killed in action at DaNang during the Vietnam War, in February 1967, one of 58,220 US military fatalities in that conflict.

Once a Crooked Man is published by Sandstone Press at £8.99