The "show insider" said:
We normally keep our guest stars under wraps but you can't hide a bloke like James. He loves coming to Wales, and it gives him a chance to meet up with Ruth Jones, who co-wrote Gavin and Stacey with him and starred as Nessa. We're just delighted to have James on board. He is a great actor and he'll be a perfect fit for the new Doctor Who.The actor's role in the series has yet to be revealed.