The musician and actor recently shot his scenes in Cardiff and admitted to PA that he got in trouble for tweeting a photograph from his trailer.
"I'm sworn to secrecy - I don't want to be killed by the Doctor Who police!" he said of his role.
"I can't tell you anything about Doctor Who. I wasn't even allowed to tweet pictures or anything, I was really upset about that."
He added: "I did tweet one picture in my trailer and I got in huge trouble with the producers on my first day so I'm not saying anything anymore about it.
"But it was amazing working with Matt Smith for a few weeks."
Walters, a former member of So Solid Crew, has had previous screen roles in Goal!, Anuvahood, Sket and Demons Never Die, as well as playing Jack Holt in BBC drama Outcasts.