A TRAIN hurtling through the dark, unsuspecting passengers oblivious to the threat in their midst. Buckle up and grab the popcorn tin, there's a new Jed Mercurio drama in town.

The Line Of Duty creator has turned his hand to BBC political thriller Bodyguard, which began on Sunday, with a stellar cast led by Keeley Hawes and Richard Madden.

Keeley plays Home Secretary Julia Montague with Richard as David Budd, a war veteran assigned as her bodyguard in his role as a specialist protection officer for the Metropolitan Police.

The opener to Bodyguard was a belter. Which should be pleasing news to the ears of former Game Of Thrones star Richard, who spent five months working on the six-part series. We first see him racing through cramped and crowded train carriages following reports of a possible terror attack.

"It was a relentless week shooting that whole sequence on a train going up and down the tracks," he attests. "It was really exciting to shoot but had its challenges because it was shot on a real train in tiny spaces. That helps add to the claustrophobia and builds that atmosphere."

Richard's voice drifts down the line from London where the 32-year-old actor, from Elderslie, Renfrewshire, now lives. He clearly enjoyed playing the bodyguard who, assigned to protect the Home Secretary, finds himself torn between his duty and his beliefs.

How much can Richard reveal about his on-screen alter ego? "We are alluding that he has PTSD [post-traumatic stress disorder]," he says. "He was a soldier who served in Iraq and Afghanistan. That is what makes it so interesting when he is thrust into a relationship with Julia.

"He's having to protect her on a daily basis when she is someone who has advocated for ground troops in Iraq and Afghanistan – something that screwed him up and he suffered from. The drama starts when you put together these two people who instantly have problems with each other."

David Budd works for the Royalty and Specialist Protection Branch (RaSP) of the Met Police. What kind of research did Richard do?

"I met a few guys who had done that job which is quite useful," he says. "With Jed you have so many great Line Of Duty contacts and advisors, so I got to meet a lot of people who had been in the police, special forces and protection officer system.

"I got to talk to them. It is quite tough because the nature of their job is that they don't talk about it and it is secret. I did a lot of work with Jed trying to build this character and make him a real guy and not just a soldier or a bodyguard."

Richard is candid when talking about the demands of filming. "To be honest it wasn't a fun shoot," he admits. "There was nothing fun about it because of the nature of these scenes and what the character was going through.

"With David, if he is not dealing with something directly in front of his face, then he is dealing with his past. I'm not going to lie, there weren't a lot of laughs on this set."

He is keen to highlight the raft of strong female characters played by Keeley alongside Gina McKee, Sophie Rundle and Pippa Haywood. "It was great to break that convention of men being in high-powered roles, even down to smaller things like our bomb disposal expert being female.

"Jed made a conscious effort to have a strong female cast and that is important, particularly when so much of the show is on my face – a young, white male.

"It was good to have pulled in a lot of different characters and it not just be very male-dominated as you can have with some police shows. Ours is definitely not like that."

There's certainly no shortage of adrenalin-pumping action in Bodyguard. Did Richard do any of his own stunts? "I did pretty much everything that I could possibly do. There were bits of stunt driving that I don't think I could be trusted to do," he says, laughing.

"On the whole, I do as much as I can. I did all my own firearms stuff and other exciting things we will see as the show goes on.

"Again, working with real officers helped because they would say: 'This is how you would hold a gun, this is how you would load it, this is how you would clear a room or search a staircase.' I learned as I went along and got enough experience to style it out."

Bodyguard continues this Sunday on BBC One, 9pm