In an interview with Hero Complex, one of the three writers on Star Trek Into Darkness, Damon Lindelof (Prometheus), sat down to talk about the importance of keeping this identity a secret.
"Benedict doesn’t even know who he’s playing," he joked.
He went on to say that "the audience needs to have the same experience that the crew is having. Kirk, you’re Spock, you’re McCoy, so if they don’t know who the bad guy is going to be in the movie, then you shouldn’t know. It’s not just keeping the secret for secrecy’s sake. It’s not giving the audience information that the characters don’t have."
He continues to say that if they found out who Benedict Cumberbatch was playing "[they] would have a five-second rush of exhilaration followed by four months of being completely and totally bummed out that they can’t tell anybody else and that when it gets revealed in the movie, it will have been spoiled for them. That’s why they’re called ‘spoilers,’ they’re not called ‘awesomes.’"
Star Trek Into Darkness is set for release in 3D and IMAX on May 9th in the UK and on May 17th in North America.