Former EastEnders star Ryan will play a "mysterious" character called Lady Christina de Souza, who joins the Doctor on a bus trip that takes "a very unexpected detour into danger".
Ryan recently guested on the first series of Merlin and headlined a remake of Bionic Woman. Last week she was rumoured to be in contention for the part of assistant to Matt Smith's Doctor in 2010.
"I'm a huge fan of Doctor Who and very excited to be joining David Tennant and the Doctor Who team," said Ryan. "It is such a fantastic show and I can't wait to get started!"
Meanwhile, Evans will appear as Malcolm, a man "whose life becomes connected to the Doctor's under extraordinary circumstances".
The special episode - titled 'Planet of the Dead' - also sees the return of Noma Dumezweni as Captain Erisa Magambo. Last seen in the series four episode 'Turn Left', UNIT officer Magambo worked with Donna and Rose to help restore the correct universe.
"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," said exec producer Russell T Davies, who is writing the episode with Gareth Roberts. "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!"
'Planet of the Dead' is the first of four special episodes airing in 2009 and early 2010 before Matt Smith takes over the iconic role.