British actress Ella Purnell takes centre stage in Sweetpea - where it’s always the quiet ones you have to look out for.

In the series, quiet and unassuming Rhiannon takes solace in her day-to-day life of being overworked and undervalued by keeping a mental kill list, just in case.

To the world, she’s a quiet wallflower – but in her imagination, she’s a force to be reckoned with and has a powerful skill for thinking up creative kills.

But these kinds of feelings can’t be hidden forever, and when her father dies and her school bully returns to town, suddenly they come rising to the surface in a spectacular manner.

Ahead of the release of Sweetpea on Sky Atlantic, Ella talks Rhiannon’s transformation, her work behind the camera as an executive producer, and what it’s like delving into the mind of a killer with a running list of potential victims.

I met with Patrick at See-Saw Films. He pitched Sweetpea to me, and I liked the sound of it. I went away and read the book and the script with the understanding that they were going to be two very different things.

I loved the tone of the books where Rhiannon is funny, she’s rude, she says what she thinks. She doesn’t seem to care, and she’s already got someone locked in the basement. I was very interested in how she got to that point. This series is more of an origin story exploring the emotional component behind the murders and the sort of addiction that it becomes.

It also seemed like the right thing at the right time. I was filming Fallout in New York, and I was looking to do something in the UK. I always had aspirations to explore roles on the other side of the camera, so when Patrick mentioned they would be open to me coming on as an Executive Producer, it just seemed like the right fit for all of us. Then I met Ella Jones, our Director, and we got along great.

One of the reasons I think the team were so open to having the actor playing Rhiannon come on as a producer is because to tell this kind of story in this kind of way, it’s really helpful to have someone on board who is solely looking at things from Rhiannon’s perspective.

In order for the audience to really get inside her head, understand and empathise with her, and hopefully go on this journey of “questionable morals” with us, I think it’s important that Rhiannon’s actions and dialogue feel as authentic as possible. I knew from the get-go that I would be good at the creative side of it: the costume, the hair, and makeup are very important to me in creating my character. So that’s something that I knew I could do.

When I came on board, we only had one episode, so I helped them shape the rest of this series, going through all of that journey, which was great, because I felt I could better understand every version of this character and really get inside her head. I knew what we wanted the audience to feel, where we wanted to end up, and the themes we wanted to explore.

Rhiannon doesn’t have to be likeable; she’s a serial killer. I’m not interested in her being likeable; I’m interested in her being relatable. Creating a relatable serial killer means that the audience is going to feel conflicted, which is what made Dexter so great, for example. So, it was nice to have the permission, the autonomy, and the collaborative spirit in shaping the show and the character.

In the series, you’re with just one character almost the entire time, and that can get annoying quickly unless the story that you’re trying to tell is rooted in this character transformation. You’re trying to tell the story of trauma. You’re trying to tell the story of grief. There’s also a coming-of-age element, which is very innocent and it involves boys. And then there’s also the fact that she is a serial killer discovering that she likes to murder people. She’s also got her childhood bully locked in the basement.

There are so many different themes and things at stake to ensure the momentum carries us through the show and also hopefully makes you want to keep watching! It’s also about finding the humour. That’s something Ella and Nick did so well; avoiding this becoming too heavy and too difficult to watch. Using humour and pacing helped to offset the natural challenges of telling this kind of story.

It’s a very British thing. We have a very dry sense of humour and a very sarcastic sense of humour. Sometimes we do use humour as a coping mechanism. I think that’s what makes our TV and our films so good, and it’s also one of the themes of my work.

Fallout, Yellowjackets, all of the things that I’ve ever done have got this survival drama element, but the thing that loops them all together is the humour. I think that it gives those moments of darkness such levity when you’re able to have a release, that breath that you need.

Casting was a really fun element of producing. I’ve never been on that side of the casting room. I wanted to read with the cast and do chemistry tests in person because it is so important that those relationships with all the characters feel different. And with almost all of them, it was just instant. We have such an incredible cast that offers such a variety of talented performances and characters.

At the Gazette, you’ve got Jeremy [Swift], Calam [Lynch], and Dustin [Demri-Burns] who are so funny. It was all comedy. Nicôle and I had a lot of fun off-set, but almost all of our scenes together Rhiannon holds so much anger towards Julia. Jon [Pointing], who plays Craig, is also a phenomenal actor. He might have been the first person we cast. We watched the show Big Boys and he is fantastic in it. He totally nailed the character, bringing a bit of that golden retriever energy, but he can also play the emotional beats.

One of my favourite scenes in the entire show is the last scene where you see Rhiannon and Craig together, and he blackmails her. We didn’t have that in place when we cast Jon. It was important to find someone who could do both of those things. He’s very naturally funny.

Each character has two very different sides, which aids in the exploration of identity and morality as a theme in the show, as well as helping challenge the “moral code” that Rhiannon develops in the show.

Rhiannon believes that there are good people and there are bad people, and that the bad people deserve to die. This black-and-white way of thinking is a trauma response she subconsciously adopted at a young age in order to process what happened to her and help her navigate the world. That’s so clearly shown in every character. Even AJ, the nicest guy, has his flaws too!

I had a very clear image in my head of what she was going to look like. I wanted to be the least ‘sparkly’ of myself. I wanted clothes that washed me out, and I wanted shit hair. I wanted it to be thin and mousey and frizzy; I didn’t want to wear any makeup. I wanted to have pale skin and bags under my eyes. I wanted to have almost that Jennifer’s Body thing that when she starts killing, she starts to become more vibrant.

By episode six, there are these subtle changes. Her hair just gets a little bit smoother, she starts to experiment with makeup because Julia’s taught her how to, and the colour comes into her face. It should subconsciously influence how you perceive that character.

The fringe is really shit, isn’t it? I love it. We tried on so many different hairstyles. It was important that after her trichotillomania recovery, she would never have touched her hair. She just let it grow out, and it grew out uneven.

Her mum left her life. Her sister left her life. She doesn’t have any friends, anyone who taught her how to do makeup or how to look after her hair, or any of those things, so it’s got split ends and it’s not in the best condition.

There’s this one line, which I love in the show, where she says, “I wanted concealer for my 16th birthday. My dad got me a grout pen.” It just so perfectly sums up Rhiannon because she probably didn’t say anything about it. She probably just thanked him very sweetly.

It was incredibly challenging, but even more rewarding. It was six months. It was a lot of night shoots, and I was in every single day, every single scene. I worked my ass off.

As well as my producer responsibilities, you’re constantly in the mindset of someone who’s incredibly insecure, incredibly afraid, very lonely, has a lot of anger, and this sense of victimhood, this sense of injustice. Being in that mindset all the time is quite difficult, so I knew going in that I would have to take care of my mental health.

In between setups and takes, I was able to just mess around with the crew, and we got close. I’d talk to Ella, hang out, lose all of Rhiannon, and just be completely myself. So I did become friends with all the people that I murdered!