Almost seven years after Happy Valley was last on screen, Sarah Lancashire and James Norton are reprising their roles as strong-willed police sergeant Catherine Cawood and hardened violent criminal Tommy Lee Royce in the third and final series of the acclaimed BBC One drama.

Set and filmed in the Calder Valley, West Yorkshire, Sally Wainwright's thrilling crime drama captures a snapshot of northern English life along with a dose of harrowing criminal activity.

This series, we find Catherine as she discovers the remains of a gangland murder victim in a drained reservoir, sparking a chain of events that leads her back to the villainous Royce - the man who raped her daughter and drove her to suicide.

Her grandson, Ryan, is now 16 and is starting to think about the relationship he wants to have with Tommy, who Catherine refuses to acknowledge as his father, while Tommy seems to have grown and developed during his time in prison - but can a leopard really change its spots?

As we look forward to the new series starting on New Year's Day, we find out more about playing a horrendous criminal and what's to come this series from the fantastic James Norton.

JAMES, IT MUST BE INTERESTING TO RETURN TO PLAYING TOMMY AFTER SO MANY YEARS. HOW WOULD YOU DESCRIBE HIM?

Tommy Lee Royce is an enigma and a puzzle and a kind of terrifying mess.

Genuinely, I am still kind of working him out. I think I had certain preconceptions about him in the first series. We would talk about him quite flippantly as if he is a psychopath.

I did a lot of work into psychopathy and was introduced to some behavioural psychologists and criminal psychologists who were really wonderfully helpful in excavating Tommy and the mindset that he has.

What we know about Tommy is that he had a very abusive childhood and lives with horrible trauma. We know that because Sally has written it into the script, and he is very, very mistrustful of the world. He sees everything and everyone as a hostility and a potential threat.

I think he feels like the way to live a happy life in his head is to be on the defensive and to attack before he gets attacked.

It's quite a sad, lonely space that Tommy lives in.

HOW DO YOU GET INTO TOMMY'S HEADSPACE?

The way into Tommy's headspace has been challenging because by nature he lives in a very different space to me and most people I know - on that kind of fringe of humanity.

But that is kind of the joy when you are asked to empathise with someone who feels very distant from yourself. That's kind of where you learn the most; if you go on that journey with empathy and understanding you get to see the world in an entirely different way.

WHERE DO WE FIND TOMMY IN SERIES THREE, AND WHAT DOES HIS JOURNEY LOOK LIKE?

We pick up Tommy seven years later and he is still in prison serving multiple life sentences. But he has changed, he has moved on and grown and he has matured. He has calmed down and there is a definite shift in his demeanour and his temperament. I think that's because for the first time in a long time he has got hope.

Unlike the Tommy we have seen of the first and second series he feels quite affable and in control, I think that's probably because we arrive in the third series and Tommy has information about something we don't know about yet. That information, that situation, is giving him a sense of contentment.

INTRIGUING! HOW DOES IT FEEL TO BE BRINGING HAPPY VALLEY BACK, THEN?

To be back here filming Happy Valley feels great because people love the show and we love making it. It's really, really wonderful knowing there's an appetite for something.

I have had so many interviews over the last seven years where people have asked me, or people in the street have asked me 'when is it coming back?', so now to be able to say we have shot a third series and it's coming back and it's as big and bold as ever is really wonderful.

WITH THE SHOW NOW COMING TO AN END, WHAT WOULD YOU SAY YOU'RE MOST PROUD OF ABOUT HAPPY VALLEY?

I am immensely proud of the series as a whole. Immensely proud that I was able to have a part in it. And it's wonderful to come back knowing there is such an appetite and a love for the show.

I was really proud when I went to America and I realised how far-reaching the show is. I think it's such a specific show about the very specific part of England, and the accents and the temperaments and sensibilities of those characters are quite specific to the area, and yet when I go to America, I am amazed at how many people have watched it and how many people love it.

That was a really proud moment, when I realised how it wasn't only a show loved in the UK but internationally.

AND FINALLY, WHAT DO YOU THINK WE SHOULD BE MOST EXCITED FOR IN THIS THIRD AND FINAL SERIES?

I think the reason this series is particularly exciting is because everyone knows it's the last, and so everyone is going to be waiting for something to happen, everyone is sort of predicting and guessing how Sally wants to end it.

I have been predicting for the last seven years how she is going to end it, so it was really wonderful to read the script and hear her ideas - and they don't disappoint.

Happy Valley's third and final series starts on BBC One, 9pm tomorrow.