Actor Benedict Cumberbatch attends the media day at the Chelsea Flower Show in London May 19, 2014.[Photo/Agencies]

Knightley, a friend of Cumberbatch's since the two worked together on Atonement, calls him "the sort of actor who never tries to simplify anything".

"If it's a complex person, he wants to dive into all the complexities and try to get all the nuances out," Knightley says. "You completely believe him in any of these roles, whether it's Assange, Stephen Hawking, whoever. He's very intelligent, but he's got a curiosity you can see and it sort of burns through his performances."

Cumberbatch, however, makes no claim to cleverness. Of Sherlock, he credits its writer: "Steven Moffat is the brain. I just say it fast."

With no footage to draw from for Turing's manner and speech, Cumberbatch met with his relatives. The actor began many of his days jogging. (Turing was an elite runner.) And he worked at crafting a plausible stutter for the famously awkward mathematician. Still, playing a man of such brainpower was challenging.

"I'm not stupid but I'm not that smart. So I can at least lend something of that within the performance, like maybe the alacrity of thought, making fast connections," says Cumberbatch. "But when you actually start talking about the language he used to get to those stunning conclusions, you might as well ask me to write my name in Mandarin."

After The Imitation Game, the 38-year-old Brit, who recently announced his engagement to Sophie Hunter, is ready for a simpler equation.

"I've done evil. I've done good. I've done smart," says Cumberbatch. "I haven't done much sexy, sexy, really. I know Sherlock's some people's cup of tea. I'd like to do a romantic comedy. I really would."

I've done evil. I've done good. I've done smart. I haven't done much sexy, sexy, really. I know Sherlock's some people's cup of tea. I'd like to do a romantic comedy. I really would."

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