Keira Knightley and Benedict Cumberbatch's portrayals of real-life World War Two code breakers took a blow on the set of their new film when they couldn't even solve a simple crossword puzzle.

The two Brits play famous encryption specialists Joan Clarke and Alan Turing in The Imitation Game, but it was clear they were just pretending to be super-smart when they decided to see if their characters' problem solving skills had rubbed off on them.

Knightley recalls, "One day we decided we should all really do the crossword. So we got the quick crossword, there were five of us, it took us five days, and we still didn't finish it. We were really bad at all of it."

And the actress admits she struggled with basic equations when a mathematics expert was brought in to help the cast become boffins: "I didn't understand any of it... It was that feeling I haven't had since school... when you sort of feel like you've died, of not being able to concentrate at all. And I desperately wanted to because he was such a nice man.

"I was thinking, 'This is really interesting, I should be paying attention'. But I couldn't."