Were you aware of the real-life events that inspired The Serpent prior to your involvement with the series?
I had no idea about the real events before filming. I had some vague recollection that there had been a documentary but I had neither seen it nor thought about it.

How would you describe your character, Paul Siemons, and the role he plays in both the series and the capture of Charles Sobhraj?

Paul Siemons is an extraordinary character and was an extraordinary person. As a character he is easily angered, but he has a very strong notion of morality and is extremely loyal. He makes a great foil for Herman. Consequently his way of doing things can help or hinder!

What made you say yes to the role?

When I read the script I found it terrifying. One rarely gets such a visceral reaction to simply looking at a script. It felt as though I'd forgotten to breathe during the reading. I immediately started reading the episodes I had been sent (you rarely get the whole thing at first) again.

Paul Siemons was different from most characters I had played: in the series he is a combination of bitter, mean, intelligent, kind, cruel sentimental, funny and capricious. Hard to turn down. Always looking for a challenge...

Did you do any research before filming?

I could find very little on the whole subject and less on Paul. I was sent information by the production team which was invaluable. His life prior to this was remarkable. He fought for the Belgian resistance during WWII, living for over two years in a forest, blowing up Nazi supply lines etc. He then became involved in what was then known as the Belgian Congo. As a result of this work he seemed to drift into the Belgian Diplomatic Corps where he was a kind of Minister Without Portfolio - officially he was not high ranking but, unofficially he dealt with 'problems' that his particular set of skills made him uniquely qualified for.

He died many years ago (he was already 60 when our story starts). I’m sorry I never got the chance to meet him. We met the real Herman during filming, which was amazing. He’s a charming man who seemed to believe that what we were doing was pretty accurate.

How did you find your time filming in Thailand?

Thailand was very hot! I thought that when I arrived in September of last year. Then it got hotter. The humidity was unbelievable. You really had to be very careful about how you expended your energy reserves. I lost track of how much water I drank every day. Basically, you drank water all the time. Amazing place though. I know it's become a cliché but the people really are incredibly kind and generous with their time and help. It is a land of permanent smiles.

That aspect made the story we were recreating even more grim. The food was, of course, fabulous. It was weird doing this job in such a beautiful place full of contradictions of extreme wealth and extreme poverty. It made the whole thing seem like a dream sometimes.

Finally, why do you think BBC One viewers should watch The Serpent?

I expect it to be a hard watch but it is so brilliantly written, directed, shot and acted that once people start I think they will be hooked and become as obsessed with the story as we were. It has the same kind of dark fascination as Chernobyl. Don't be put off by the darkness though, things this well done don't come along too often. It's important to face that side of human nature and see how it can be confronted.