This was mainly because, whenever she wasn’t working, she was plugged into her iPhone. It was the very latest model, the iPhone 8, in rose gold with a protective cover that sparkled with crystal glitter. She played games – Minecraft and Monument Valley – and she was forever checking her Twitter account. I never actually heard her speak to anybody, but she was endlessly texting, obviously in a relationship with someone. Her phone would ping in the middle of a scene and drive Ewan Lloyd to distraction. She would apologise sweetly even as she was firing off a reply. I had never seen anyone’s thumbs move so quickly.

Nothing about her quite connected. For example, the sweatshirts and leggings she liked to wear had come straight out of Sports Locker, but she also had a Cartier watch and her shoes were Jimmy Choo. She talked about popular culture – Star Wars and The Hunger Games – but I noticed her reading Franz Kafka. The playlist on her iPhone included Björk and Madonna, but finding a piano in the rehearsal room, she sat down and played the first bars of a Bach prelude. I was quite sure there was something she wasn’t telling us.

That left Tirian Kirke, playing Mark Styler. He had come in late, replacing the actor who had decided that five months on the road was enough for him, and I’m afraid he was the only cast member I didn’t warm to … but then he and I had history.

A couple of years older than Sky, Tirian had made many appearances on TV, playing a junior case officer in Spooks, a police constable in Line of Duty and, for three seasons, a footman or some sort of under-butler in Downton Abbey. He wasn’t quite a household name, but he was well on his way to becoming one, so I had been very pleased when he was cast in a show I’d written called Injustice. This was a five-part legal drama starring James Purefoy, screened by ITV in 2011. It was also, coincidentally, the show where I first met Hawthorne. He was our technical adviser.

Tirian was going to play a young offender who falls foul of the prison system and eventually ends up taking his own life. It was a really good role. The character had four or five hefty scenes, plenty of screen time and a memorable death. He’d done a great audition and had been offered the part almost at once. He had accepted. The contract had been drawn up. But then, at the last minute, he had changed his mind. According to his agent, he had decided that the script wasn’t good enough, which hardly endeared him to me. The part was eventually played by Joe Cole, who did a brilliant job and went on to become a major star – but that didn’t change my feelings about Tirian. He’d wasted time and money. He’d let us down.

So I was nervous when Ewan told me that he had been cast as Mark Styler. Firstly, I was afraid that my earlier experience might repeat itself, but more to the point, Tirian came across as rather too pleased with himself to play the part, too self-conscious, with his carefully groomed hair, his designer clothes and the Ducati motorbike he drove to rehearsals. I had to remind myself that all of this had come with success and that he was, at heart, a very good actor and we were lucky to have him. The first time I saw him reading the part, he even looked exactly how I had imagined the character: slim and bony, with dark eyes and an unusual, angular face. He had a crooked nose and a lazy, roguish smile, far removed from the anodyne good looks of many young British actors. He was the sort of actor you couldn’t help noticing – and this was exactly what had happened. He had come to the attention of the Hollywood director Christopher Nolan, who had cast him in a big-budget production – Tenet – which was due to start shooting later in the year.