It's already possible to predict the one major cause for complaint that will assail the new series of Fifties-set Grantchester... No topless river scene.

However, the ever "unlikely" crime-solving partnership of Robson Green (Police Inspector Geordie Keating) and James Norton (vicar Sidney Chambers) are not in the least disgruntled at the prospect of keeping their clothes on.

"I'm not so disappointed - but I would have gone in!" says London-born Norton, 31, wryly.

Robson Green in Granchester on ITV

The pair's bare chests were available to ogle in series two of the hit ITV show - written by Daisy Coulam and based on the books by author James Runcie - and their 'Mr Darcy moment' is still cause for a certain amount of rivalry between the two leads.

"James was shivering away in that river scene! I just dived in really quickly because I wasn't going to compete with that body!" says Green, 52, with a laugh.

Instead, this new six-part series picks up where the Christmas special left off. Sidney's forbidden love interest - divorced, single mother Amanda (played by Morven Christie) - has given birth to her baby, Grace, and the almost-couple are trying to work out exactly what they're doing.

"He's kind of exploring this thing with her, a not-quite relationship, no one's sure what it is. His relationship with Grace is confusing because he's there as a sort of father figure, but not a surrogate father, so it's typically confusing and complicated," summarises Norton.

The crux of this series, he explains, is love versus duty.

"We're really going into depth about: What is a vocation for anyone? It's quite an odd concept for a modern audience. So few people go into the church nowadays," says the actor, who studied theology at Cambridge and considers himself "fascinated" with religion. "In the Fifties, lots of people would; one son would go in the army, one son would go into the church - and so the calling was not as rare as it is now, so it's quite hard to empathise sometimes with Sidney's choices, because they're so rooted in this vocation.

"A lot of us are going, 'What are you doing? Surely love conquers all?' But he keeps getting battered around the head by his faith. What I would do, and what Sidney would do, is very different, but that's the beauty of the writing, that's what's so much bigger than his love affair - his faith. What he begins to doubt is the institution of the church, but actually, his relationship with God is totally unwavering, and it's too big, too powerful."

Grantchester on ITV - James Norton

Meanwhile, Geordie, who appears happily married to Cathy (Kacey Ainsworth), is in fact struggling with his feelings for police secretary Margaret (Seline Hizli).

"On the face of it, you find him well, but underneath, the only person who knows things aren't that well is Sidney," says Northumberland-born Green. "And Sidney's going through his own dilemma, so they try and give each other advice - which is a beautiful thing - but they can't because they're not qualified. No matter how they try, they'll always fire the bullet back and say, 'Well, you can't say that because you're corrupt yourself!'"

[Read more: Morven Christie - There's nothing sexy about kissing James Norton]

Expect the mismatched duo to continue to wrangle with their dysfunctional friendship, in between the odd murder, of course.

"The likeable, very unlikely, very endearing relationship between Sidney and Geordie, it's what drives the show," says Green, who's starred in TV hits Soldier Soldier and Wire In The Blood. "There's an osmosis between the two of them. It's so rare in your career that you come across that.

"[Sidney's] duty is to God, it defines who he is, whereas Geordie truly believes you cannot solve anything by confiding in an invisible friend. You have these opposing views but this inherent need for each other."

While Geordie and Sidney may be struggling to pick the right path, for Green and Norton, this series has been their most comfortable and effortless yet.

Green recalls shooting a scene together a few hours before this interview, which took just one shot to complete and director Rob Evans had said: "I'd only go again because it'd be different, not because it'd be better."

"Me and James are at that stage where we know each other's timing," explains Green, "we know the shorthand, and there is a genuine love there in front of the lens."

Grantchester on ITV - Robson Green and James Norton

He's adamant he wouldn't do the show if Norton wasn't involved.

"Usually I'd be off by now. I get cabin fever, I'm a terrible sufferer of that in terms of, 'Right I've done it!'" the actor admits. "But the scenes aren't the same and the writing isn't the same. I only do it because James is in it, and Daisy's writing - that's the remit."

The problem is that RADA-educated Norton is massively in demand right now. He's recently starred in Happy Valley, War & Peace and McMafia, and is appearing in upcoming film Flatliners, alongside Kiefer Sutherland and Ellen Page.

"He's charismatic, he's incredibly likeable, he's engaging and he's a fine actor. He's going to fly," says Green graciously. "I hope he does, he's so hard-working."

Fortunately, despite calls from Hollywood, Norton can't shake the intellectual and emotional pull of Grantchester either...

"The only representations of faith [on TV] are often quite comic, or sinister and quite negative," Norton muses.

"What I really enjoy is that Grantchester's a bit different to all those representations of Christianity, and it shows a man who has a very pure and simple faith, and doesn't really make it funny or dark or twisted in any way - and he's a hero. I'm proud of that."

Grantchester returns to ITV on Sunday, April 23 at 9pm. Set it to series record on your BT box now and never miss an episode with BT TV Catch-Up and the ITV Hub app.

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Grantchester: All you need to know - Cast, special guests, spoilers

Grantchester on ITV

James Norton and Robson Green return as the unlikely 1950s crime fighting duo, Vicar Sidney Chambers and Police Inspector Geordie Keating, in the third series of Grantchester, produced by Kudos.  The six-part series is based on The Grantchester
Mysteries by James Runcie.

Tessa Peake-Jones is back as Sidney’s sharp tongued housekeeper Mrs Maguire who keeps the vicar in check; Morven Christie reprises her role as Sidney’s forbidden lover Amanda; Al Weaver returns as the timid curate Leonard Finch; Kacey Ainsworth is back as Geordie’s long suffering wife Cathy, Lorne MacFadyen puts his police helmet back on to play DC Phil Wilkinson and Seline Hizli returns as the feisty police secretary Margaret. And of course it wouldn’t be Grantchester without the return of the gorgeous black Labrador Dickens.

In the third series of Grantchester new Archdeacon Gabriel Atubo (Gary Beadle) reminds Sidney that as a clergyman he must put duty above his own needs and lead by example. Given the strength of his feelings for Amanda, is this something that Sidney can do?

On the face of it, Geordie is still happily married to Cathy. But there continues to be an undeniable spark between himself and Margaret.

Leonard was left heartbroken after Daniel (Oliver Dimsdale) left him for another man, but in this series he appears to put this behind him and find romantic happiness with someone else. But will those around him allow him this happiness?

Mrs Maguire’s friendship with dashing Jack (Nick Brimble) develops, but can anyone ever replace her late husband Ronnie?

The heart of the show remains the friendship between Sidney and Geordie. As they solve crimes in Grantchester, Cambridge and beyond, they each deal with personal crises at home, and once again their unique and unlikely friendship is put to the test.

Peter Davison will guest star in the series as Geoff Towler, the village solicitor, captain of the cricket team and a man who rules his family with a rod of iron.  Further cast includes Charlie Higson, Lorraine Ashbourne, Adrian Bower, Susannah Harker, Charlie Dale and Holly Aird.