He has kept quiet about the details, but has commented on the role to RadioTimes.com:
"'Would you like to be in Doctor Who?' is the easiest question I've ever been asked. It's a 35-year dream come true. Seven-year old me would be going off his nut and I'm not far behind.
"It's a great way to spend a couple of weeks, working with people I've always wanted to work with on a show I've loved all my life. My bucket list is quite a lot shorter now."
He joins Michelle Gomez, Sanjeev Bhaskar, Jemma Redgrave and Ingrid Oliver in the series finale.