Jenna-Louise Coleman has promised to bring a "feisty" edge to her role as the new companion in Doctor Who.
The actress, who will appear with Matt Smith and Richard E Grant in the Christmas Day special, plays barmaid-turned governess Clara.
The episode, written by show supremo Steven Moffat, is set on Christmas Eve 1892 and sees the Doctor up against an army of snowmen led by the villainous Doctor Simeon played by Grant.
Moffat admitted he had written the episode with a typically festive "noisy, sugared-up, slightly tipsy household" in mind.
Jenna-Louise said her character was "a mysterious one", saying she was "very down to earth, but feisty and curious too".
She said: "She isn't intimidated by the Doctor, instead she finds him amazing and ridiculous. But she is on her own mission and lives by her own means, she is very resourceful."
Grant, who has twice played The Doctor - in online and spoof versions - said he did not mind being cast as the villain this time.
He said: "When you're born with a 10ft long face, you don't get hero roles, but I'm not complaining as I have hugely enjoyed the wide variety of parts I've got to play."