How would you describe Bill and how does he fit into series two?

I play Lieutenant Colonel Bill Stirling, David Stirling’s older brother. He is the head of their aristocratic Scottish family and so is used to being an authority figure in a position of responsibility. At the end of the first series we see David has been taken prisoner of war and now the regiment is left in limbo with Paddy Mayne in command, and Paddy is a loose cannon. The future of the regiment is in doubt because the upper echelons of the British Army despise and resent it in many ways. Bill comes on board to try and save the regiment. He tries to do his best to help Paddy Mayne with guidance and some firm words to ensure the future of the SAS – which doesn’t go down particularly well with Paddy. The journey of the series is about Bill trying to find the best way to manage Paddy, and to harness his potential. Bill isn’t naive – he trained commando units for years before we are introduced to him in the series, so he is well accustomed to people like Paddy Mayne and recognises his brilliance as a warrior. The challenge is how to hone and control that temperament, which becomes a difficult learning curve for Bill across the series.

How does Bill differ to his brother David?

Bill is described in the show as being the brother who didn’t fall out of a tree and hit his head when he was younger. He is perhaps more serious and diplomatic than his colourful younger brother and is somewhat more of an enigma. His approach to leadership is totally different to David’s. First of all he’s not so much on the battlefield – he plays more of a strategic almost political role. Bill is the liaison between GCHQ and the regiment and he is constantly going back and forth to GCHQ, defending the SAS and discussing how they should be used. During this period the SAS are used as shock troops – special formations created to lead military attacks – much to Paddy’s disappointment. But what Paddy and the SAS don’t know is that behind the scenes Bill is grafting to change that and is dealing with some very powerful people in the shape of Generals Dempsey and Montgomery. He does all of this quietly and determinedly without fanfare as he wasn’t a man who was concerned with legacy or notoriety, he just believed in the SAS and its survival.

What was it like to joining the cast and become part of the SAS Rogue Heroes family?

I’ve loved being embraced by them, it’s a really special job. It can be tricky coming into a second series with a really established cast of actors and a great crew, as you worry about ruffling feathers or breaking into a clique, but there was none of that whatsoever. It was open arms from day one.

It’s an incredible story we’re telling about some really brave and courageous people, and we all feel that weight of responsibility. There is a passion in everything we do in order to do justice to this story.

What do you think viewers will enjoy most about series two?

It’s a different period of the war, therefore it immediately has a very different feel to it. We’re moving away from the desert and into mainland Europe and with that there are encounters with civilians and resistance groups. The SAS is being taken in a different direction in terms of having to act more as a conventional commando unit, but it still has this rogue-ish element to it with this wild bunch of warriors. You still have that anarchic energy which is ever present.

You start to see the effects of war on individuals in this series. You really see the toll it takes on them mentally and physically.

What were the standout moments of filming for you?

There was so many, it’s been such a joyful job. My first day on set was pretty standout because I was just buzzing to be a part of it, and it was a moment of action for Bill. It was Bill and 2SAS charging into battle for the first time as this new regiment to act as reinforcements. I was in the full battle gear speeding down a dirt track in a jeep with a gun by my side and shouting to charge through a check point. That was very exciting! Me and Stuart Thompson, who plays Greville-Bell, were doing our best not to just scream like little giddy children because the adrenaline was pumping and we were far too excited.

I loved grappling and getting to lock horns with Jack O’Connell as well. He is a class act and I’ve always loved watching his work. He’s just up for it, he’s up for the scrap and that’s a really exciting place to be – not knowing what’s happening next, being on your toes with another actor.

Spring, 1943. Paddy Mayne takes control of the SAS following David Stirling’s capture, as attention turns from the conflict in North Africa to mainland Europe. But GHQ have cast doubt over the future of the regiment, while the creation of a second unit and an influx of new arrivals make things even more difficult for the men. Can they prove that the SAS remains essential to the war, wherever it may lead them? Based on Ben Macintyre’s best-selling book of the same name.

SAS Rogue Heroes series two is available in full on BBC iPlayer from 6am on New Year’s Day, and airs on BBC One from 9pm that night.