Just the idea that someone is married and they've got a kid, and he reports for work one morning and his boss says, "You're wife is a spy. Shoot her." In the real story, he just went back and did it, which would have been a short film. Therefore, I had to spend some time exploring what you would do.

It's great that the story [Allied] is set in the '40s because the '40s feel to it is completely appropriate.

What happened was I was invited to meet Tom [Hardy] to discuss a project that he had in his mind about an adventurer who returns to England from Africa with secrets and with a history, and the original idea was set some 80 years later than it is now. But in the conversation I really took to the idea and I'd wanted for a while to set something in 1830 and 1840 in London, so it struck a chord.

What I wanted to do [in Allied] was get two characters who fall in love for real, across the barricade, and then it transcends the war.

I've been writing so much. And what happens with TV is that they split [Taboo] into two blocks, so you get a director that does four and another director that does another four. You commit yourself to seven days a week, for 12 or 13 hour days for a long time. I couldn't really do that.

It has to be an actress like Marion Cotillard [in Allied] because there are so many levels to it. It's set in the Second World War, when lots of people were doing things that, outside of a war, you wouldn't do, like killing and dropping bombs. She's doing things that one wouldn't approve of, but it's war.

Obviously, television is a writer's medium, so you get a lot more power and authority. With a film, the discipline is having a beginning, middle and end, and having it work in a specific space of time.