What has happened with the Daleks since we last saw them?
"The last survivor of the Cult of Skaro, Dalek Caan, has had a very rough time. Something very odd has happened to him! That's linked up to a load of stuff that I couldn't possibly reveal because unfortunately the Daleks have used a mind probe to erase my brain."
When did you find out that you would be returning for the final two episodes?
"I think those who I know that knew about it, including David Tennant, might have given me hints pretty early on. I think David said to me, 'I'm not cagey but if things go as planned, you'll probably be here in March for the final block.' I'd kind of known it was on the cards for quite some time."
How did you react when you read the script for this week's episode, 'The Stolen Earth'?
"Well first of all, it didn't have a title, so I didn't know what was [going to happen]. It's a great, exciting title, but it does rather give stuff away, doesn't it? The whole build-up was very exciting, bringing together the new mythos of Doctor Who with Torchwood and Sarah Jane. I was quite taken aback about how epic it is - and it really pays off in the final episode, which of course I can't tell you anything about. D'oh! How annoying."
How difficult was it to perfect the voice of the deranged Dalek Caan?
"Well, it was a great challenge, a bit like when we did the Dalek Emperor in the first series. When Russell [T Davies] introduces a character like that, there is a whole chunk of stuff in the script that describes the voice. He did the same for Dalek Caan, which gave me a real key into what I could do. Then I evolved all these different theories about Dalek Caan's neurons firing in the wrong direction in his brain. He can't tell when he's happy or sad, his emphasis is very strange and he finds things funny when things aren't funny. He's a kind of soothsayer really. His mind is almost pure."
Do you ever get to improvise any Dalek dialogue?
"No, although there's some bits in this one where they kind of go a bit crazy, and things go wrong at one point, so they do a few strange noises. I suppose that required improvisation in a scene. Also I remember in 'Parting of the Ways' when they flew into the void, I had hours of fun in the studio [doing Dalek screams]. When I heard it back, it was hilarious, because they'd multitracked it. It sounded like the cat in that old public information film!"
We know Davros is returning for the finale, but are there any other foes from the classic series you'd like to see come back?
"I do the Doctor Who audio adventures for Big Finish, and we've had the Ice Warriors back in a couple of times. I've rather enjoyed doing those. I can't immediately think of anything else that should come back. I'd be happy for it all to be Daleks, frankly."
What was it like on set with so many former companions returning?
"It was like a really good party, in that people would all be standing around in different groups chatting, and everybody was getting on really well. There was a slight 'end-of-term' feeling, in the best possible way. I suppose you would wonder if there might be some tension, but there wasn't at all. It was lovely to see people back you haven't seen for a while."
How do you feel about Steven Moffat taking over as executive producer?
"Steve's take on Doctor Who is fascinating. His stories have won awards. I'm really excited about it. It'll be interesting to see because he's got such an interesting mind. I don't know what's coming up at all, but I suppose he's going to change a few things."
Finally we know the Cybermen are back for this year's Christmas Special. Are you back doing their voices?
"Well, were the Cybermen to be involved in something, I guess they would get me to do the voices! If that is in fact true. I believe there may have been some pictures of the Cybermen on the internet..."
Doctor Who airs Saturday at 7.10pm on BBC One.
Additional reporting by Ben Rawson-Jones.