Cumberbatch is 'a handful', says The Imitation Game director Morten Tyldum

He has charmed audiences everywhere with his take on Sherlock Holmes and has become one of the most sought-after actors on either side of the Atlantic. But what's it like directing him? The director of his new film, the Imitation Game, says he can be challenging.

is challenging, says the director, Morten Tyldum, of his new film, and 'a handful', according to his co-star.

Even he will admit that, given his way, he'd probably still be perfecting scenes from The Imitation Game instead of promoting the movie this week at the Toronto International Film Festival.

Such is the curse of working with , one of the most sought-after actors today on both sides of the Atlantic. Last month, he won his first Emmy for his role as a modern-day Sherlock Holmes in the BBC television miniseries Sherlock, which airs on PBS in the United States.

Now the 38-year-old Englishman is among early favourites for an Oscar best actor nomination for his portrayal of Alan Turing, the unlikely British World War Two hero.

In , Turing is the brilliant mathematician who breaks the Germans' Enigma code, helping to bring the war to an end and save millions of lives. He took his own life at 41 after he was convicted for being a homosexual.

Tyldum said Cumberbatch, his first choice for the role of Turing, proved himself 'a challenging actor because he expects so much from you'.

"When I would say 'we've got a phenomenal take here, we got it, let's move on,' he would say 'just let me try and do something different'," said Tyldum, a Norwegian directing his first film in English.

"And he does something even better, that I could not even think of."

Keira Knightley, who plays Joan Clarke, the lone woman on Turing's code-breaking team of math, chess and puzzle experts, said her 'very old friend' is unrelenting in his quest for great work.
"He's always a handful," said Knightley. "He always questions and is very, very passionate and committed to making it as good as it can be and to not cut any corners. That is what he is like in life and that is what he's like to work with."

Cumberbatch said he wanted to give Tyldum a choice when he went to edit and could have filmed all day.

"I'd still be filming it now if I had my way," he said.

But Cumberbatch said he tries to be mindful as the lead actor not to use up everyone else's time and exasperate his co-stars.

Matthew Goode, who plays dashing leader of the code-breakers Hugh Alexander, forced to step aside for Turing to take over, said he's learned to 'batch' with his friend Cumberbatch.

"He points a finger at himself, saying 'I can batch myself more'," said Goode.

"And we say: 'If you are going to batch yourself a little bit more, I am going to batch in'."