The casting directory lists the two actors as appearing in episodes five and six of the new run, written by Bonekickers creator Matthew Graham.
The episodes, currently titled 'The Rebel Flesh' and 'Gangers', will see Smart play a character named Jennifer Lucas. The actress previously starred in BBC detective drama Wallander as Anne-Britt Hoglund, and has also appeared in At Home with the Braithwaites alongside former Doctor Who star Peter Davison.
Lancaster, known for his role as Chris Skelton in Life On Mars and sequel series Ashes To Ashes, will play the role of Buzzer in the two-parter. He has also featured in multiple episodes of Coronation Street as Slug and recently made a guest appearance on Doctors.
It was announced in September that Lancaster's Ashes co-star Daniel Mays had been cast in a forthcoming Who episode.
Filming on the fifth and sixth episodes is currently ongoing in Cardiff.
The Doctor Who one-off episode 'A Christmas Carol' will air on Christmas Day on BBC One in the UK and BBC America in the US. The sixth series is to follow in spring 2011.