Inspiration, as every amateur detective knows, can strike at unlikely times and in unlikely places. In the case of Father Brown - a ten-part TV drama based on the classic stories by G. K. Chesterton - it struck at 11.30 on a May morning, on Radio 4.

Deep in the inner recesses of the BBC, producers had been trying to come up with a new crime show that could run in the daytime schedules. Various options had been proposed. Nothing quite hit the mark.

Then, in May 2011, one of them happened to switch on a radio documentary about Chesterton's most famous literary creation, and a figurative lightbulb popped into life.

"The little Essex priest" with "a face as round and dull as a Norfolk dumpling" had just the right credentials for a pleasing murder-mystery serial in the Miss Marple mould. He was unassuming, and yet something of a maverick. He was quirky, but brilliant. His setting was a hazy but comforting yesteryear.

He had also enjoyed a long and relatively fruitful relationship with celluloid: a 1954 film, starring Alec Guinness, and a 1974 TV series, with Kenneth More. More to the point, he was out of copyright.

"We had done a lot of research into what kinds of crime shows worked on daytime television," explains the executive producer, Will Trotter, whose CV includes the weekday medical drama Doctors and the Afternoon Play strand. "I had previously done Land Girls, which was a period show, and that had worked well.

"So the sense was: not contemporary. And it just so happened that Ann Widdecombe had been doing that documentary, mentioning that some of her favourite books were these stories by Chesterton."

The Controller of LBBC Daytime, Liam Keelan, thought that the idea was good. "So then it was about how we could make that work - how we could reinvent it."

Father Brown first saw print in September 1910, in a short story called The Blue Cross. He had been marinating in the writer's mind for some time before publication.

In late 1903, Chesterton had taken a walk across the Yorkshire moors with a Roman Catholic curate, John O'Connor: "a small man with a smooth face and a demure but elfish expression". In the course of their ramble, Chesterton revealed that he was working on a crime story concerning "some rather sordid social questions of vice and crime".

The curate's response surprised him. "To prevent me from falling into the mare's nest, [O'Connor] told me certain facts he knew about perverted practices which I certainly shall not set down or discuss here," Chesterton later recalled in his autobiography.

The episode was shocking on two levels. First, because the apparently worldly-wise writer had been jolted out of his naïvety about the moral capabilities of men. "I had not imagined that the world could hold such horrors," he wrote. Second, because the jolting had come from, of all people, an unworldly priest: "It was a curious experience to find that this quiet and pleasant celibate had plumbed those abysses far deeper than I."

It struck Chesterton that most laypeople made precisely the same mistake about clerical credulity, and the idea for Father Brown was born. "I permitted myself the grave liberty of taking my friend and knocking him about . . . punching his intelligent countenance into a condition of pudding-faced fatuity, and generally disguising Father O'Connor as Father Brown," he wrote.

UNTIL Father Brown came along, the market for amateur sleuths had been dominated by Arthur Conan Doyle. But, while no criminal could be unaware of Sherlock Holmes's manifest genius, Chesterton's particular genius was in making Brown the sort of innocuous brainbox whom everybody failed to notice or, if they did, suspected of being a nonentity.

In The Blue Cross, Chesterton constantly invites the reader to giggle at Brown's outward aspect. The narrator describes him as having "a mooncalf simplicity" and "eyes as empty as the North Sea". The principal villain, the fabled criminal Flambeau, calls him a "bumpkin" and a "celibate simpleton."

When the tables turn and Father Brown is shown to have been in complete control of the story (and Flambeau) from start to finish, the revelation hits the reader like a thunderclap. "Has it never struck you", Father Brown asks Flambeau, "that a man who does next to nothing but hear men's real sins is not likely to be wholly unaware of human evil?"

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The character proved to be a huge hit, and Chesterton continued to write about him up to his death in 1936.

THE charm of the stories is found, in part, in the deft balancing of high seriousness with Father Brown's comic turns. This is something that the producers have attempted to capture with the casting of Mark Williams, an actor and comedian, who has The Fast Show and Mr Weasley from Harry Potter among his credits.

"Mark is a really good actor in terms of the range that he can do," Trotter explains. "He can do comedy, and he can do really intelligent character acting.

"What you don't want is a two-dimensional comedy priest; that would be really uninteresting. But, at the same time, you need someone who can be a bit irreverent, because Brown is a maverick in the way that he deals with the world."

Williams agrees. He was even responsible for restoring one specific example of Brown's maverick qualities into the adaptation of the Blue Cross mystery. "In the original story, there's a moment where Father Brown throws some soup up a wall as he leaves a restaurant." It was a nice detail; so they added it in.

"Those are the kinds of moments that I like," Williams explains, "where he does something very shocking in terms of social behaviour. We tried to get a few of those in there."

Something else that he was keen to retain from the books, he says, was the spiritual animus that imbues Brown's detective work. "I didn't want to make any apologies for the fact that he's a deeply committed Christian who solves murders," he explains. "As far as I'm concerned, Father Brown is Father Brown. His vocation is intense, and I wanted to respect that."

This is well observed. Father Brown's vocational intensity is the thing that distinguishes him from most other fictional detectives. Sherlock Holmes may be said to be driven by the thrill of the game,

Miss Marple by the riddle of human nature, and Hercule Poirot by the notion of justice. Father Brown's motivation, by contrast, is eternity.

"With him it's not just a question of solving a little puzzle," Williams says. "The fact that somebody has committed a crime also means that their soul is in peril. So when he knows who the criminal is, this guy is riven by it. There's 'blood in the sky'. The world becomes a terrible place because he can see that.

"So there's a hugely religious element to what he does. He does it to save souls. It's not like Miss Marple, and that: 'Oh well, that's all right then; let's have a cup of tea.' He will make somebody confess to something, knowing that they are going to go to the gallows."

Williams is coy about his own beliefs. "I wouldn't describe myself," he says simply. "I think it's counterproductive and will be used, and I'm not going to mention it."

He is slightly cagey, too, about the research he did for the role. But his reading appears to have been extensive ("Everything I could get hold of on the Catholic Church, pre- and post-Vatican II") and seems to have taken in Cardinal Bernardin's "seamless garment" theory of a consistent life-ethic. His aim, he says, was to get under the bonnet of Brown's belief system. "He's obviously a very intelligent man, and I wanted to know what his manuals and documents were."

Williams also spent some time with the show's script consultant, Fr Gwilym Lloyd, a parish priest in the archdiocese of Birmingham. Fr Lloyd offered the actor a few tips on authentic clerical practice. "He lent me his little folding pocket stole," says Williams. "It's a kind of spiritual emergency kit, which I thought was wonderful. I also wanted to ask him about how you deal with the dead, what obeisance you would make for a dead body that had just been stabbed, for instance - that kind of detail."

IF Williams's portrayal cleaves closely to Chesterton, many of the show's larger set-pieces are quite different. Brown's locale, for example, has been relocated to a village in the Cotswolds. Trotter is candid about the commercial reasons for doing so. "The environment that Chesterton set him in was London, but the stories jumped around all over the place. To make that work on the amount of money that a daytime budget allows meant that one had to find a precinct, if you like, where the stories could be told.

"The other thing is that BBC Worldwide [the Corporation's commercial subsidiary] co-funded this series. They're looking for a global sales market. What they want is to show off England across the world, and the Cotswolds is a perfect area for that. First, it's timeless: you can almost shoot through 360 degrees once you've moved the cars out of the way. And, second, it has these amazing houses - quite moneyed and full of antiques and looking lovely - that we can tell the mysteries in."

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An even bigger tweak involves the time-zone, swapping the Edwardian and inter-war period for the 1950s. The reasoning here was to move the series "away from that Art Deco/Poirot world", Trotter says, while allowing the writers to append modern themes. "Things like nuclear threats and thalidomide," he suggests as examples.

Will Chesterton fans baulk at these changes? Possibly, Trotter, says. But he thinks that the author would approve. "I assume that, if Chesterton were writing in the 1950s, he would put [those kinds of storylines] in. He was not unaware of social issues."

Besides, he says, TV constantly reworks the classics. "Are the Conan Doyle purists happy with the new Sherlock?" he asks, referring to the BBC's modernised Sherlock, which has won nearly universal praise from Canon Doyle fans.

The trick is to capture the spirit of the source text, Trotter says, and, in that sense, Father Brown is a faithful rendering, he reckons. "In its essence, we're still saying the same thing: it's a Catholic priest who seeks redemption in people.

"I mean, the purists will say: 'It's not set in the right time, he should be more passive, more thinking. . .' I don't know: they could come up with all sorts of things. But, ultimately, it gets Chesterton's character on the screen. And that's the most important thing."

Father Brown starts on BBC1 on 14 January at 2.10 p.m.