The multi-million selling author's new novel, Simple Genius, revisits familiar themes of secrecy and whodunnit-style plot twists starring the former Secret Service agents turned private sleuths, Sean King and Michelle Maxwell. So far, so formulaic. But in perhaps the biggest twist of all, this time around Baldacci adds depth, going so far as to send Michelle to a mental institution with a case of OCD.

"She's a character I care a lot about. I have found that people who are super overachievers will never allow themselves to be outworked and will exceed even their own potential . . . often, there's something behind that."

This surprising narrative turn . . . making your lead fallible . . . is effective and realistic, betraying Baldacci's own brush with mental illness. "I had some experience with that with the people I care about. It is devastating. It's chronic and it really can destroy your life if you're not careful. But there's usually some underlying reason."

"The fact I like [Michelle] so much as a character . . . for her to have this flaw, it was difficult to write. In a way, I think it made her more human."

If you were cynical, you might think Baldacci's use of OCD might just be down to another device . . . after all, people will eagerly keep reading to find out what Michelle's dark secrets are. However, it's not his only cause celebre.

Baldacci and his wife have also set up a literacy foundation, and he is an ambassador for the National Multiple Sclerosis Society: "My sister has multiple sclerosis and we decided it's a good way to raise awareness."

Like his genre-mate John Grisham, Baldacci qualified as an attorney and practiced for nine years in Washington, DC. He wrote short stories in his spare time and eventually graduated to screenplays.

After growing frustrated with the Hollywood system . . . "producers would challenge me on every word" . . . Baldacci started writing novels.

"I have found that going through that dissective process made me a better writer because it makes you think about so many things, " he said. "Underlying character motivations, for instance, and plausability. Motivation is such a key thing when you're writing . . . particularly thrillers, because if you have somebody doing something outrageous, you have to justify that."

After the tremendous success of his first novel Absolute Power, a story of presidential cover-ups, Baldacci silenced his script critics when Clint Eastwood approached him about making it into a film.

The experience wasn't quite what Baldacci had in mind.

"We went to the LA premiere and we were sitting in the audience and Clint Eastwood got up and introduced me.

"We sat down and started watching the movie. And the first act was straight from the book, and I was thinking 'Oh my God, this is fantastic'. But then, because of the paramaters Eastwood had given the scriptwriters . . . 'I'm the hero. I don't die like my character does.' . . . the screenwriter, Will Goldman, had to re-write the second and third acts totally. Everything after the first act . . . much of it is not even in the book. And the ending is completely made up.

"I kept squeezing my poor wife's handf I just wanted to scream, 'This is not my stuff!

The first 20 minutes yes, but this is not my stuff!'" He laughed. "But you get over that. It's apples and oranges."

Baldacci's good humour comes across in his books, along with the realism that shows a writer who has researched his topic. Simple Genius deals with quantam physics, cryptography and piano prodigies, telling the story of a code-breaking centre across from a CIA training ground called Camp Peary. "I found the force of details that gives authenticity makes people really believe the stakes are genuine.

"You layer detail after detail and wed it to things really going on, like the CIA . . . Camp Peary is a true facility. I just thought it was a kick to have one super-secret facility here and, across the river, another."

As for one true-life code to crack, what advice does he have for would-be novelists?

"Ask yourself why you want to be a writer. The answer should not be 'because I think I can make money fast'. The answer should be 'because I love reading.' Be ever-curiousf Eavesdrop shamelesslyf you never know when ideas can strike you. You'll know if you're a writer because you will imagine . . . if that door opened, who might be standing there given a certain set of circumstances that I would have to create? If they're going to walk in, what might change in the world?"