Airing at 9.30pm on Saturday 19th March, the one-off drama explores Isherwood’s formative years in 1930s Berlin in the run-up to the Second World War and co-stars Lindsay Duncan (Doctor Who: ‘The Waters Of Mars’), Imogen Poots (28 Weeks Later), Toby Jones (Doctor Who: ‘Amy’s Choice’) and Douglas Booth (Worried About The Boy).

Smith told BBC Press Office: “For research I went to see Don Bachardy [Isherwood’s partner before he died] in the States at the house they shared. Don helped me understand his character – he said Christopher was very polite. Just seeing the love Don had for him, and to be in the space where Christopher had lived and written was very informative.”

He continued: “I also read Christopher’s novels, and then spent a lot of time trawling through video footage, watching, listening and repeating his voice. I had a wonderful voice coach called Patsy Rodenburg who said: ‘Look, just go for it. Don’t worry about trying to imitate him. What’s more interesting is trying to capture the essence of the man and then be yourself.’

Smith added: “Christopher was such a wonderful writer of prose. The other thing I learnt from Don is Christopher would always write in the morning, he’d get something done every day. When I was playing Christopher I would write as much as I could every day.”