ITV's Grantchester is back, and, while the new episodes will see James Norton make his exit from the show, Robson Green is here to stay as Detective Geordie Keating. Georgia Humphreys finds out what the fourth series has in store.

Robson Green isn't one to mince his words.

As the 54-year-old, who was born in Northumberland, discusses why he loves his role in ITV's Grantchester, he blurts out that other acting jobs have made him "deeply unhappy".

"There's a couple of things I've done where I've walked off, because I just can't abide the people I'm working with," he confides.

"I mean, narcissism is part of the equation of being an actor, but my God, there's different levels and scales, and I've been alongside most of them.

"But here [Grantchester] genuinely makes me happy. And I get paid for it. That's a terrific thing to have in your life."

The detective drama in question, set in the Cambridgeshire village of the same name, is about to return for a fourth series.

It's now 1956, and "things are changing in that very likeable world of Grantchester", as Robson, who plays gruff, down-to-earth police inspector Geordie Keating, puts it.

"It's moving far too fast for Geordie," continues the talkative star, most famous for portraying Fusilier Dave Tucker in Soldier Soldier and Dr Tony Hill in Wire In The Blood.

"And he wants to go back to how it was, the good old days, which is quite relevant to today ... Whatever you think of Brexit, the argument is the feeling there are too many corrupt minds within young people. You know, 'There used to be discipline, and integrity, and dignity and you knew your neighbours' and all that."

The cast of Grantchester is going to look a little different in the new episodes too.

James Norton, who plays charming clergyman Sidney Chambers, is leaving at some point during this series, while Outlander star Tom Brittney, 28, is joining as the young and confident Reverend Will Davenport.

The friendship between crime-fighting pair Geordie and Sidney has undoubtedly been a huge part of the show's success.

Was Green worried about what it would be like without Norton by his side on screen?

"I think everybody was, deep down," he admits.

"But I was fortunate enough to do the chemistry reads with the actors who were going to replace James, way before the show was even starting filming.

"As soon as Tom came in, it was immediate. We just went, 'Oh, that's him. That's the guy'."

As for his relationship with McMafia star Norton off screen, Green says: "First and foremost we're friends, we're kindred spirits. So that element's never gone."

"But he doesn't reply to texts!" he adds in jest. "He's terrible.

"But my God, when he texts me, he wants a reply ... I was in Australia, three o'clock in the morning, and he's texting away, 'I need to know this, I need to know this'. Well, f*** me, I texted you the other day and you didn't reply!"

Green is smiling throughout all of this chatter, and it's evident he and Norton have a lot of laughs together.

"The strange thing, for me, was saying goodbye to the laughter, the joy, the intense relationship we had within this very short period of time, every year, for four years," he continues of his co-star's departure from the drama.

"It was intense. We were together most of the day, every day, more so than you spend with your partner, or your parents, or your family. And I enjoyed every minute, I never wanted it to stop. But, you know, nothing good ever lasts, and he had to leave."

Luckily, Brittney's character promises to be a very exciting addition to the show - Will is described as caring and a man of the people, who puts his all into a quest for social justice.

But he also has a troubled past and, as the series goes on, more about his story, and perhaps why be became a vicar, will be revealed.

For now, father-of-one Robson gives an insight into Geordie's relationship with the new vicar.

"Will is very opinionated, thinks God is above the law, so that really puts Geordie on the back foot.

"But we've developed this relationship where he's the son I never had, and I'm the father figure he never had. And that's really nice to play."

New themes will be explored this series too - the civil rights movement, racism, homophobia, domestic violence and sexual assault.

What was it like to tackle such storylines?

"It's really important," notes Robson.

"But I don't see it as, 'Right, we're going to talk about racism in this episode'. It's how racism affects the relationships. To me, that's how good drama should be played out."

Robson later points out how relevant the issues - and how they're dealt with - still are in the world we live in today.

"You can draw parallels, and you ask yourself, 'Has anything really changed?'" he suggests.

"And I think to a certain extent it has, because we're talking about it more, especially domestic violence, and racism we've spoken about a lot, homophobia is spoken about.

"With the new movement, and more reporting of sexual assault, especially in the workplace, and at home, that's coming to the forefront. And quite right.

"Therefore, the themes within 1950s Grantchester are relevant to today, so people can relate to them. And that makes it very watchable.

"But again, we don't beat the audience over the head with the themes. They're very beautifully structured in the writing of drama."

Grantchester returns to ITV on Friday January 11