When did you find out you would be coming back for David Tennant's final episodes?
"Quite late on. I'm not sure when they decided but I think I was only told a month or two before filming."
How did you feel?
"Oh, so thrilled! I was always hoping I would be brought back in some way so it was a brilliant feeling."
How is it possible? The Master storyline felt like it was all wrapped up.
"I know but then that's the beauty of Doctor Who, isn't it? The whole regeneration idea and all these twisty sci-fi rules. Anything can happen in sci-fi!"
The last we saw, The Master's ring was being collected from his ashes. Can we assume you were picking up his ring?
"It wasn't me! You're going to have to keep watching to find out who it was!"
Lucy shot The Master, which led to his apparent death. What's her relationship with him like this time?
"Not good. She's been incarcerated since, as a punishment for the shooting, so she has a lot of time to reflect on how bad it all got. She's a little bit of a reformed character."
Where was she incarcerated?
"I'm not 100% sure to be honest. She's been incarcerated in a really epic castle dungeon-style prison which we actually shot in a medieval castle in Wales - it was freezing! I'm not quite sure who incarcerates her but there's a governor of the prison - a new governor... that's as much as I can tell you."
When she's let out is she angry or has she found peace?
"She's actually not necessarily let out! She's quite an angry person, I would say."
Does she have a bigger role in the story this time around?
"No, not really. I would say a similar kind of significance to the last one. It's definitely more The Master's story."
John Simm's changed his hair. Do you prefer him as a blond?
"That smacks of 1992 grunge, so no I don't!"
There's some great guest stars in the specials, like June Whitfield and Bernard Cribbins. Did you get to act with them?
"No, I didn't. Because I've been incarcerated, my scenes were more with John and a few of the other actors. There were a lot of actors there I didn't get to work with."
Everyone wants to know what happens - what was the security like on set?
"So top secret it was unbelievable! We weren't even given the last few pages of the episode. Anyone who was in them obviously had them but everyone else wasn't allowed to know how it culminated. When you auditioned - not that this was an audition - you were only sent your own scenes so it's very, very secretive."
Is that frustrating?
"I like it actually. When you read a script it's totally different to when you see it, especially with some sci-fi stuff. I really love being surprised when I watch it. It is a bit frustrating but it's probably best for interviews because actors can give things away when they don't mean to, so it's probably a really good idea - Russell knows actors too well!"
Before you got the part were you a big fan of the show?
"I was. I missed all the original Doctor Who because I was too young but I was aware of it and I was quite excited when I heard it was being brought back - partly because I was a big Russell T Davies fan actually. I was really excited, yeah."
What's your opinion on David leaving and the new regime?
"David is definitely my favourite Doctor but I think that Matt will be brilliant. The whole point of Doctor Who is that lots of different actors play him so it's totally fitting and appropriate. It was time for a new Doctor, wasn't it?"
Finally, are you done with the show or might you come back?
"You know what? I have no idea! I would say anything is possible but I don't know the new team. I don't know how many characters they're resurrecting from the new series - probably not that many - so I don't know."
'The End Of Time' airs Christmas Day and New Year's Day on BBC One. US viewers can watch the two-part special on December 26 and January 2 on BBC America.
Who do you think collected The Master's ring? And how do you think he comes back? Add your comments to this entry below!