When Nicholas Searle retired from the civil service, he pursued his dream of becoming a writer by taking an online course. He was taken aback when publishers snapped up his first novel and turned it into a bestseller in 2016. Now he has had the “surreal” experience of watching Sir Ian McKellen and Dame Helen Mirren bring his characters to life in a major Warner Bros film adaptation of that debut book.

The Good Liar, a thriller full of twists and turns, tells the story of ageing conman Roy, who preys on a wealthy widow, Betty, after meeting her online. Searle, who describes himself as “60, but a baby in the writing business”, told the Observer that watching such a dream cast flesh out his characters has been extraordinary.

“[They] came from the vacuum of my mind,” he said. “[Then] there they are, on the screen, as if in real life. It’s a rather surreal experience… It was reassuring, but quite a shock just how living and breathing on the screen Roy and Betty were.”

“Perhaps it’s me as a star-struck author.”

He added: “The performances are just so strong. They bring the whole thing to life. When one reads the screenplay, one sees the gaps, but one doesn’t realise quite how the actors and the director fill the gaps between the lines. That’s the absolute magic of acting.

“Betty in the book appears at the beginning to be quite a light-hearted, almost ditzy character, but all the time – and this was my aim, at least – you’re conscious that there’s rather more to her... That’s exactly what Helen brings to the part.” The film is directed by Bill Condon, who worked with McKellen on Gods and Monsters and Mr Holmes. Searle said: “I’m absolutely flabbergasted, both with the two stars and to have such a strong director.”

He began his novel on a course run by Curtis Brown, the leading literary agency. Once finished, he submitted it to the agency’s joint chief executive, Jonny Geller,whose authors include John le Carré.

Geller responded to it immediately. Describing it as “absolutely brilliant”, he said: “I sold it to Penguin and around the world and it went on to sell 100,000 copies in paperback last year. So it did really well.”

He added: “It’s about an elderly man who finds love on a website… A slightly oddball romance begins. Then you realise something’s just not right about this… There are loads of twists. It’s very clever.”

It is all a world away from Searle’s previous career, which he will only describe as working “in the sphere of security and intelligence”.