Jeany Spark on the "traumatic" past of key Collateral character Sandrine Shaw

• Meet the cast of Collateral
• Carey Mulligan: film is "lagging behind" TV when it comes to creating good roles for women
• Collateral writer David Hare: I was "scared" by prospect of making full TV series

"I suppose it is unusual for a classic whodunit to have the reveal in episode one," actress Jeany Spark – who plays Sandrine – tells RadioTimes.com. "But with David Hare’s writing it’s never going to be a classic whodunit, it’s always going to be more of a 'whydunit' – and because I was quite familiar with David’s work before, I knew the interest and the meat of the piece was going to be exploration of character and motivation and social politics more than was it the butler in the pantry with the candlestick."

That exploration of character and motivation comes to the fore in episode two as we learn more about Sandrine and her chequered past. For Spark – best known for playing Greg Davies' on-screen love interest Emma in Man Down – taking on a nefarious character was a welcome departure.

"To be playing a baddie in a black rubber balaclava – it's kind of living the dream. They're quite cathartic to play because you get to go to work and behave in ways you'd never behave in real life and get it out of your system. And then you come home and go, 'Oh right, I've been a murderous killer for the day so let's make some tea'."

Although – as Spark notes – Sandrine is not a straightforward "murderous killer". A special forces captain as part of the Special Reconnaissance Regiment, it's her past trauma that has led her to be consigned to a desk job at a British army base.

"We're going to be seeing a lot more of Sandrine over the next three episodes, finding out more about who she is, what's motivating her and what her situation is.

"Although she's committed this murder which from the outside looks very immoral and unethical, she's actually an incredibly moral, ethical military woman who lives her life by a code of honour. But unfortunately her code of honour isn't serving her terribly well so there's that tension."

Sandrine's compromised morality might be connected with the events of her recent tour of duty which led to the deaths of some of her closest colleagues.

"She's not long returned from action in Afghanistan which was very traumatic for her," explains Spark, "so she's quite vulnerable and damaged by what she's been through on and off the battlefield. In episode two you really see her struggling with trying to get on with her life and return to some sort of normality within the army and in her personal life as well. You realise it's not as straightforward as a cold-blooded military assassination."

To effectively portray her character's experiences, Spark did her research. "This is the first time I've played anyone in the army so I was keen to do as much preparation as I possibly could. I did a military-style boot camp with a group of ex-military consultants and military advisors, learning to talk the talk and walk the walk, but I was also able to talk to them about their experiences. They were able to advise me about what it's like to lose people who are your responsibility to take care of and then to have to live with that and carry on. Some extraordinary men and women told me about their experiences, their friends' experiences, and were very open about it, very candid. I owe them an awful lot for that."

Sandrine is also a victim of unwelcome sexual advances from her boss, something Spark watched documentaries to prepare for. "It's a very difficult subject. In any institution unfortunately abuse of power is quite a common occurrence so I had limited discussions with ex-military about that."

Anyone who watched episode one will know that Collateral – which also stars Carey Mulligan, John Simm and Billie Piper – is very issue-led, dealing with immigration, poverty and homosexuality in the church, as well as sexual harassment. It's a wide reach that Spark says is one of the drama's strengths.

"I think a state of the nation thriller which explores these very current, very topical hot issues is fantastic and I like that it's not clear cut. There are no definite obvious baddies and goodies."

She adds that the series "looks at these institutions that we place our faith in, seeing their limitations and how much should we question them, how much should we allow them to lead us.

"I think it's fascinating and timely, particularly in the wake of Brexit, to reexamine all these establishments and what they mean and how they operate. I've got a lot more questions and a lot more things to think about."

Collateral continues on Mondays at 9pm on BBC1