The Tunnel started as a French / Anglo remake of Nordic noir The Bridge. Then from season two it struck out on its own and headed into new territory but was still anchored by the offbeat relationship between the two lead detectives Karl (Stephen Dillane) and Elise (Clémence Poésy). He's emotional, compassionate but steady, she is awkward but logical and meticulous, together they balanced each other out and formed an unlikely bond.

Season 3: Vengeance takes inspiration from Brexit and the refugee crisis for a timely thriller with a political edge. We caught up with Dillane and Poésy to discuss the third and final season.

How does Karl start working with Elise again?

Stephen Dillane: At the beginning of the first episode there's a burnt-out boat brought into the harbour. It transpires there's been some strange event on this boat to do with people trafficking. The boat came from France and is now in England, so they get together to work out what's happened.

What's their relationship like as we start season three?

Clémence Poésy: They're very close now. Even from the beginning of season two, there was that worry of where do we take it now there's not that antagonism at the start? How do we make it work? We start from a point where they have each other's trust, so the interesting thing is to find the bumps on that road that would unbalance things enough for it to be an interesting journey towards each other again. She starts the season wanting nothing to do with Karl because she's been separating different elements in her life to try not to fuck up again. She's made decisions that she knows Karl wouldn't approve of, having decided to go back to her old job. She's kept him quite far away.

SD: Karl's a bit taken aback at first that Elise has been demoted and seems non-communicative. But having got over that, their relationship is very familiar. They accommodate each other easily, so when information is withheld, it's a surprise and slightly distressing, and means something. Elise does need a little looking after because her mental state seems a bit fragile.

What do you see as the key themes in The Tunnel: Vengeance?

SD: The relative invisibility and value of people. Are some people worth more than others, and if so can that be helped? How hard should we work to ensure that we use our imaginations to value everybody equally?

CP: It was quite interesting to see how the writers would address Brexit. The decision has been made to be quite metaphorical, although there are a few brilliant lines about it. The idea I think was that just by watching Elise and Karl work together, against the fact that their bosses are doing all they can not to collaborate, was their way to say: 'it's not that bad, we were trying to stick together'. As for terrorism and the refugee crisis, when we started season one it felt like all the seeds were there and things had happened, but it [feels] like a different world to now. It's mad to see how things have accelerated in four or five years.

SD: It's a tricky thing because it's such a fast-moving story. With TV programmes scripted so far in advance and in the knowledge they'll be transmitted so much later, it's difficult to be absolutely current, but Brexit is acknowledged. It exists in conversations between characters, but its political implications aren't really investigated.

What do you think sets The Tunnel apart from other thrillers?

SD: I hope there's a more off-centre way of storytelling, so the dominant Hollywood thriller narrative style is knocked off centre a bit. It's less about lurching from thrill to thrill – although the thrills are there, of course – and more to do with relationships. It's a more humane kind of storytelling – or that's my perspective, from the inside.

CP: It's told a friendship story which is quite rare, especially between male and female characters. In most stories, those relationships always seems to become sexual or romantic at some point. I'm quite attached to Karl and Elise's relationship. I find it quite moving.

The Tunnel: Vengeance starts on Sky Atlantic, Thu 14 Dec, 9pm.