Fan mail from the 12th Doctor.
Other interesting tidbits that have come out:
* Capaldi auditioned to be the 11th Doctor, but wasn't chosen
* He was the only auditionee for the 12th Doctor
* Apparently Smith wasn't the first choice for the 11th Doctor. According to Neil Gaiman, a black actor was offered the role but turned it down. Gaiman won't say who, but I'm going to guess Lenny Henry.