Alien star Hurt appeared in the BBC sci-fi drama's May finale - credited as 'The Doctor' - and will return in the forthcoming 50th anniversary special.
The Telegraph now quotes Who costume designer Howard Burden as saying that Hurt is portraying a previously unseen "dark Doctor".
"There was a gap between Paul McGann playing the Doctor and Christopher Eccleston, when we didn't see a regeneration, and John Hurt will fit into that gap," Burden's quotes read. "He is a past Doctor, not a future Doctor."
The notion of Hurt playing an 'in-between' Doctor was already a popular theory amongst Doctor Who fans. The idea first surfaced when the 73-year-old actor was seen on the set of the 50th special sporting a leather jacket similar to the one worn by Eccleston's Ninth Doctor.
The anniversary instalment - also starring Matt Smith and David Tennant - will air on BBC One on Saturday, November 23.
> Doctor Who and the big finale question: Who is John Hurt playing?
Watch John Hurt's first appearance in Doctor Who below: