In The Serpent, an eight-part series based on Sobhraj’s real-life crimes, Coleman stars as his French-Canadian lover, Marie-Andrée Leclerc. Adopting the aliases Alain and Monique, the pair befriend western tourists, drug them, and steal their passports and money. Those who fight back are quietly disposed of by Sobhraj (played with icy conviction by Tahar Rahim) and his associate Ajay Chowdhury (Amesh Edireweera). Hot on their heels, however, are Herman and Angela Knippenberg (Billy Howle and Ellie Bamber), a Dutch diplomat and his wife who are investigating a string of unsolved murders, as well as Sobhraj and Leclerc’s suspicious neighbour Nadine Gires (Mathilde Warnier). The result is a globetrotting nailbiter with spectacular ’70s fashion to boot.

As the show continues on BBC One, Coleman tells us about meeting the real Knippenberg and Gires, reading Leclerc’s diaries and the cheesy playlist that helped her get into character.

‘The Serpent’ is based on an unbelievable true story. When did the project first land on your desk?

“I was sent the script while I was doing All My Sons at The Old Vic [in 2019]. It was a story I’d never heard about before. They also sent me lots of photographs, Marie-Andrée’s real-life statement and recordings of her voice. I was hooked. She was mysterious and complicated, and the more I read about her, the more addicted I became. It’s a story that’s seductive and intoxicating, but also dark.”

How much does the real story of their crimes diverge from what we see on screen?

“A lot of it is obviously our interpretation [of events]. For example, we don’t know and may never know to what extent Marie-Andrée was an accomplice or a victim. But much of it is based on facts. A friend messaged me the other day saying, ‘Was the monkey really poisoned?’ So, I texted [one of the show’s writers] Richard [Warlow] and [director] Tom [Shankland]. They replied, ‘Fact!’ There’s so much in the show that you wouldn’t believe, but that did actually happen.”

How did you want to balance those two sides of Marie-Andrée — the victim and accomplice?

“It’s something I had to wrestle with. I’d never encountered anyone like her before in terms of her psychology, and her background was really interesting. She was from a religious family and when she writes about her life [in her diaries] there’s a greyness and banality to it. She was a romantic and her self-esteem was low. She meets Charles Sobhraj and within three weeks she was drugging people. We don’t know how much she truly knew about what else Charles was doing, but he almost became like her religion. She allowed her moral compass to be totally buried.”

Is it true that you met the real-life Herman Knippenberg and Nadine Gires during filming?

“They came to set and I think it was strange for them. I asked Nadine if Marie-Andrée and Charles were really in love. She said, ‘To begin with, yes, and Marie-Andrée always.’ But, she also said that she’d asked Marie-Andrée if they were going to have children and apparently she’d replied, ‘I couldn’t. He would be a monster.’ So, she was aware of that and yet she wouldn’t leave him.”

Marie-Andrée is, of course, French-Canadian. Did you learn French from scratch for the show?

“I did! It was crazy because I was doing the play and we only had a few weeks before we started shooting, so a lot of it became about the muscularity of the mouth and the sounds. It was a language I’d never learned, but there was no way I wasn’t going to do the show. I’ve also long been a fan of [French-Algerian actor] Tahar Rahim [who plays Sobhraj]. I saw him in A Prophet [2009] in my early twenties. And so, within a few weeks, I was on set with Tahar speaking French.”

The ’70s fashion in the show is so glamorous. How did Marie-Andrée’s look come together?

“We had lots of reference images and spoke about wanting a dark [Brigitte] Bardot vibe. In my mind, Marie-Andrée was obsessed with magazines, particularly Vogue Paris. The idea was that she wanted to shed her French-Canadian identity and transform into somebody else. There’s a floatiness to her look and I brought the idea of her wearing kimonos around the house. The sunglasses are incredible too, and then there’s what she wears in Paris. We looked at images of Mick and Bianca Jagger, and that’s where we got the idea for [Sobhraj and Leclerc’s] suits. I was also part-wigged because when we first meet Marie-Andrée she has quite big curly hair.”

What other resources helped you to get into character?

“Tom Shankland, the director, sent me a Marie-Andrée playlist. We played with this idea that she’s got bad taste in music and liked cheesy romantic songs. The playlist had John Denver’s Leaving on a Jet Plane, ABBA’s Mamma Mia, Elton John’s Tiny Dancer and Simon & Garfunkel’s Bridge Over Troubled Water. There’s also a scene where something horrible is happening, but Marie-Andrée is in the next room listening to Charles Aznavour’s She (Tous les visages de l’amour) to block it out. We see that she wants to keep playing this romantic heroine that she has in her head.”

The show hops from Bangkok to Kathmandu to Paris, but was it mostly filmed in Thailand?

“It was mainly in Thailand, but we travelled around a lot. The Dal Lake scene, for example, was shot near Myanmar. We were meant to fly to Budapest to shoot the Paris scenes, but that’s when Covid hit. We were flown home and there was a six-month break. We couldn’t really travel after that, so we shot the rest of the series in Tring in Hertfordshire, which acts as part of Mumbai, Karachi and Paris. It’s been such an adventure, especially in Bangkok with the markets, the traffic and the heat. Our Thai crew were amazing, and the set was this melting pot of languages and cultures. It really captured the hedonism of the ’70s. Now, to have the show come out after all that, is thrilling.”

In terms of what you have coming up next, is there any more news on ‘Victoria’ season four?

“Not at the moment. I do have something else coming up, but I’ve signed an NDA so I can’t say more, and then hopefully a few indie films as well once everything starts getting back to normal.”

How have you spent your time in lockdown when you haven’t been filming?

“It feels like so much time has passed. I saw someone post an Instagram about banana bread the other day and I thought, ‘Wow, we’re back to banana bread again!’ [laughs]. I’ve been learning French still, cooking, gardening and I started doing a photography course with Leica. The highlight of my social life has been meeting friends for walks. Zoom quizzes got boring very quickly.”

What did you learn from 2020 that you’ve taken with you into 2021?

“We need to slow down. There’s a feeling that you always need to be achieving something or pushing forward, everyone’s diaries are chock-a-block and we try to multitask on such a level that we forget the beauty of just surrendering. I’ve appreciated the stillness, and also the kindness. I feel like there was so much of that around at the beginning of lockdown. I hope that continues.”

The Serpent is now airing on BBC One and available to stream on BBC iPlayer