'But what she really wants from him is to be told to stop f***ing around, not be mollycoddled and treated as damaged goods.' The make-up department helped with the actor's physical transformation, dyeing his naturally ginger hair blond, padding out his tall, slender body, and stuffing his long, hollow cheeks with plastic plumpers.
'I had Boris Johnson as a visual reference,' Cumberbatch says, retracting his angular face into his neck and tugging at his jowls. but I think they were slightly nervous about any kind of attractiveness being completely lost.' It is a dull July afternoon and we are talking over lunch in an airy restaurant in west London.
Cumberbatch speaks in sentences of extravagant length and confidence.
'The tragedy is that Tietjens and his wife love each other, it's just the wrong kind of love, expressed in the wrong kind of way,' he says.
His Tietjens is the kind of man who will pore over the Encyclopaedia Britannica, scribbling corrections in the margin, while the nation stands on the brink of war, and his wife (Rebecca Hall) – a volatile beauty who is openly cheating on him with another man – hurls insults at him across the breakfast table.
'He tries to kill her with kindness,' Cumberbatch says.
If Sherlock zips breakneck across the screen like a jet ski, Tietjens glides, with the stately movement of an ocean liner.
Where Sherlock arrives at the truth by employing what Cumberbatch calls his 'sociopathic, motormouth deducting genius', Tietjens seems to carry it within him.Benedict Cumberbatch has fallen in love with an overweight civil servant called Christopher.'I am enamoured of his principles, his virtue and the goodness he stands for,' he says, grinning the wonky grin of the hopelessly smitten. I really do think that Christopher Tietjens is the character I'm most fond of ever having played.' Tietjens needs all the love he can get.'Perhaps you didn't even think about how you felt.' Cumberbatch's performance, in an ambitious series co-produced by the BBC with the American network HBO, is mesmerising; it is further proof of his knack for making even the most peculiar characters seem overwhelmingly human.While his Sherlock Holmes, the character that made him a household name and bona fide heart-throb, may share a certain cruel wit with his Tietjens, in other ways the two figures could hardly be more different.'There but for the grace of God a lot of us might go, or are going, or have gone.' The role of the cerebral maverick is not untrodden ground for Cumberbatch.