Shaun of the Dead star Kate Ashfield on why she has made her first TV drama about a teenage psychopath

Now she has written her first drama, Born to Kill, an almost unimaginably dark four-parter coming to Channel 4 later this month.

It's set in a featureless commuter town and follows a teenage psychopath, a manipulative and damaged individual called Sam (played by newcomer Jack Rowan) who lives with his well-meaning nurse Mum (played by Romola Garai). Sam’s Dad is in prison (for something horrible, we can assume, though we don’t find out in episode one) and he harbours powerful urges to kill.

Ashfield herself is a single Mum to a (very well adjusted and lovely of course) 13-year-old boy. And she came up with the idea at the school gates with her friend and writing partner, Silent Witness scriptwriter Tracey Malone.

"We were both fascinated by why people do what they do and the darker side of life,” she explains. “We wanted to explore a teenage psychopath and not what you generally see, which is an older person.

“There are plenty of children that kill and when it comes to your mind you see it more and more. There are quite a few cases; it’s awful but it happens.”

Sam is a charming, plausible and smiley individual who helps out at the hospital where his Mum works, reading to elderly patients. But he struggles to form friendships and, like the older people he befriends, can only mimic the social interactions of others, consistently regarding everyone he meets as tools for his ego.

Ashfield and Malone spent hours researching the drama and there is little about psychopathology she doesn’t know, talking freely about pleasure glands, brain nuclei and predispositions to psychopathy. And, while she is a mother herself, she is not afraid to take a hard, long look at the darkest side to adolescence.

“My friend’s Dad used to call her son Idi Amin,” she laughs. "They can be little emperors; they want everything their own way. We would say a child is autistic but we wouldn’t ever call a child a psychopath. We would say they had callous and unemotional traits.”

Ashfield describes Sam as “a wolf in sheep’s clothing” and believes people like him are not as rare as you might imagine.

“There are far more people like him walking around who don’t kill than those who do.”

There is another question that occurs to me though – why isn’t she in it?

She says that she didn’t feel comfortable casting herself because she didn’t want to be seen to be writing a vehicle for herself.

Also, she probably wouldn’t have had time, discovering that she and Malone were working hard on her scripts even after filming started.

“I loved writing. It’s hard work. But I really enjoyed it and go from being an actor to the other side and see how the whole thing works.

"When you go for an audition you see there are these people in front of you and you have no idea how that all came together. Even being in meetings where you are meeting different directors and so on, you see that directors go through the same thing as we do, selling themselves.”

It seems like she will be selling herself a bit more in future. She will continue taking on acting jobs, but she has a number of writing ideas bubbling away and a handful of scripts in development.

If they are as savage and compelling as this she will be a writer - as well as an actress - to be reckoned with.

Born to Kill starts on Channel 4 on Saturday 20 April