Michelle Ryan and Lee Evans will guest star in the Doctor Who Easter special, Planet of the Dead.
Former EastEnders star Michelle plays the mysterious Lady Christina de Souza, who joins the Doctor on a bus trip which takes a ‘detour into danger’, the BBC said.
Comedian Evans plays a character called Malcolm, whose life becomes connected to the Doctor’s under extraordinary circumstances.
Ryan said: “I’m a huge fan of Doctor Who and very excited to be joining David Tennant and the Doctor Who team. It is such a fantastic show and I can’t wait to get started.”
There has already been speculation that Ryan could be cast as the Doctor’s new assistant, starring opposite new Doctor Matt Smith.
Planet of the Dead is the first of four Doctor Who Specials which will air in 2009. David Tennant will bow out as the Doctor at the end of the year.
Noma Dumezweni returns as Captain Erisa Magambo – last seen helping to save the world in the episode Turn Left.
Executive producer and writer Russell T Davies said: “Michelle is one of the most sought after young actors in the country and we are delighted to announce that she will be joining the team.
“As always the script is being kept strictly under wraps, however we can reveal that Lady Christina is a woman with a mysterious past who’s going to have a huge impact on the Doctor.”