'The characters are much more richly developed,’ Radcliffe says. 'The young doctor is on a steep moral decline, but the key is to keep the humour. We pay appropriate homage to the moments where you need to be sufficiently dark or sad. But the psychology and behaviour of an addict is pretty laughable, and there is a lot to be made fun of in that insane logic.’

'I do prefer working to not working,’ he says. 'Growing up on the set of Harry Potter, I spent an incredible amount of time filming things. One of the lovely things about my work since has been the pace I get to move about. You get a sense of momentum. In this we would knock off six scenes a day. There is such a sense of achievement.’

'The script would say, “Dan sings in Italian,” or, “Dan breaks out into a funny dance.” I don’t speak Italian, and the only funny dance I know I plagiarised from How To Succeed. I think the writers took great pleasure in seeing what Jon and I would do. But I have a lot of trust in them. So if they have me dancing in a clown costume, I know it will be for very good reasons.’

'You wouldn’t think of us as looking the same,’ Radcliffe says, 'but we can be made to look the same. I think Jon and I try to let some of each other in. A tiny fraction of his phlegmatic thing has come into my performance, and a little bit of my mania has seeped into him, so we are starting to share the same personality traits.’

'I owe everything to Potter, but playing a character for so long gives you this pent-up energy and desire to try as many different things as possible. The films I made last year and A Young Doctor have been huge leaps forward for me. I think I am getting better at knowing what I am good and not good for,’ he says.

'Primarily I am trying to test and challenge myself and to get better, because that is what excites me the most. But if I can get just one more person to read Bulgakov, or to come to see The Cripple of Inishmaan, that is incredibly cool.’

'I have never worked in a situation like this where there is constant dialogue between the writers and cast,’ Radcliffe says. 'We were always throwing new things in, and there was a lovely collaborative atmosphere.’ Hamm describes the show as finding a 'creative sweet spot’.