Andrew Davies has said the incestuous liaison between Helene and Anatole Kuragin is "crucial" to his adaptation of War And Peace.
Dickensian star Tuppence Middleton and Glue's Callum Turner play the siblings who are seen in bed together in the new version of Leo Tolstoy's famous novel.
Talking to the Radio Times about his writing process, the 79-year-old Welshman said: " I often say I just 'copy out the best bits'."
However, Davies acknowledged it is not always apparent what the "best bits" may be.
" For instance, in War And Peace, brother and sister Helene and Anatole are in an incestuous relationship, but Tolstoy indicates this so subtly that most readers, including me, at first reading, miss it altogether."
Davies continued: "But this relationship, and their attitude to it, is so crucial to our understanding of them that for me, at least, it needs to be on the screen."
The hotly-anticipated BBC One drama boasts an impressive cast: Downton Abbey's Lily James, Grantchester's James Norton, Harry Potter's Jim Broadbent and The Fall's Gillian Anderson are among notable names involved.
Davies, who wrote A Very Peculiar Practice and children's series Educating Marmalade, is also famous for adapting the acclaimed 1990 BBC version of House of Cards starring Ian Richardson.
But it was the huge success of Pride and Prejudice in 1995, starring Jennifer Ehle and Colin Firth, which cemented his reputation for fine television adaptations of classic literature.
Davies has since worked on Vanity Fair, Bleak House and Sense And Sensibility, to name a few.
However, he had never read War And Peace until he was asked to consider adapting it for TV by Faith Penhale, head of drama for BBC Wales.
"I took it with me on holiday, and you know what? Once you get into it, it's a page-turner," he said.
"Great characters, thrilling action scenes, lots of battles (of course), a classic duel, several love stories, with seductions, betrayals and a happy ending for some, but not all, of our favourite characters. What's not to like?"
War And Peace is broadcast on BBC One on Sunday at 9pm.