Well the secret is a secret no more, the twelfth Doctor will be played by 55 year old Peter Capaldi, who is quite well known as an actor.
He replaces Matt Smith, who at 26 was one of the youngest actors to play Doctor Who, and Peter is the same age that William Hartnell was when he started to play the role of the Doctor. Peter will replace Matt in the Christmas special, and there is no news if Jenna Coleman will continue as the Doctor’s companion.
It will be interesting to see how an older actor will interpret the role.