Can you first tell us a little bit about the Christmas special?

The Christmas special for me is a bitter-sweet episode because I'm leaving, but Steven [Moffat] has written a brilliant, adventurous, funny episode and I'm really thrilled with it. It feels wonderfully Christmassy.

What did you want from your last episode?

I think it's good for the Doctor to go out with a bang, a crash and a wallop. I'm pleased it's really funny and mad. When I got to the last 20 pages and it was quite a hard read for me, but I hope it's going to be a belter.

We've got a great director in Jamie Payne and some really lovely double-hand stuff with me and Jenna [Coleman]. Steven's managed to tie in plot points and narratives that have been threaded through over years and I think that's ingenious.

Emotionally, how did it feel to be doing your final performance?

It felt very emotional to be doing my final episode. My mother is mortified, honestly she was at the front of campaigning for me to stay and wasn't happy when I said I was going to leave. But, when you've got to go, you've got to go.

Of course, it's very sad for me in many ways because everything is the last time. It's the last read-through, the last time I put on the bow tie and the last scene in the TARDIS. But the show is about change and I had lunch with Peter Capaldi shortly after the announcement and I think he's just going to be incredible. He has the most brilliant ideas. As a fan, I'm genuinely excited to see what he's going to do because I think he's going to do something extraordinary.

So you're still going to be a fan?

Yeah, absolutely. I'll be a fan. I'm very grateful to Steven Moffat and that whole team up in Wales for the past four years and you know I want the show to go from strength to strength, which it will. It might take me a couple of weeks to get my head around it. I think it was the same for David and I think it was the same for Karen, when she watched Jenna come in. I don't think it's easy, but it's not my show, it's the fans' show, so I'll be a fan and then it will be my show.

Have you been given anything by the fans as a leaving present?

I was doing a promo shoot for the 50th and these two girls came on set. They had made me this book which must have had 50 or 60 letters in, saying thank you for Doctor Who, for being part of it. Things like that are amazing. I've said it before, the fans of this show are really spectacular and they've made this an extraordinary journey for me and I'm very, very grateful. I don't think there's another set of fans like it.

Can we expect some nods to past series with this Christmas special? Are we referencing old specials?

With Doctor Who you're always looking back and forward at the same time, because you tend to be jumping around. I don't want to give too much away, but obviously when you look at my tenure over the last four years, there are stories and plot points and villains in there that are particular to my Doctor and I think he's got to face all that.

Can you set the scene for this Christmas episode?

It's his final battle and he's been fighting it for a while. The Doctor is facing the joint challenge of a mysterious event in space that has summoned lots of aliens to one place and helping Clara cook Christmas dinner. There are also elements from every series of Matt's Doctor, which will come to a head in this special. Things that we've laid down for years are going to be paid off.

How was the read-through?

It was emotional. I think possibly the beginning of the end is more emotional than the actual end. It was the same with "The Angels Take Manhattan", when Karen and Arthur left. The read-throughs are the moments that tend to get people because obviously the shoot dissolves into what we hope will be a tremendously exciting wrap party.

Did you know what you wanted Matt's last words to be?

I didn't think I would go that way, but a couple of months before I wrote it I did say to Mark [Gatiss] that I thought I knew what his last moment would be. And indeed his last line. But if it didn't fit the scene I wouldn't crowbar it in. I've had the vague storyline in place for a long while.

What episodes or scenes do you think will define Matt's time as the Doctor?

I think "The Eleventh Hour" was such an extraordinary debut. Everybody for a year of poor Matt Smith's life had been saying, "Total mistake. He's far too young." Then he came in and he was brilliant. "Vincent And The Doctor" was also such a lovely episode and I was thrilled Richard Curtis was able to write for the show. There's the physical comedy that Matt has brought and of course fish fingers and custard. I think his relationship with his own TARDIS in "The Doctor's Wife" was gorgeous.

What do you think distinguishes Matt from the other Doctors?

I think he does old Doctor better than anybody else. It's not an accident. It's something he very, very consciously thought about. Because he was the youngest Doctor Matt said, "He's only got young skin. Nothing else is young." I think Matt makes you think very believably that he is this ancient being.

The Christmas special will introduce the next Doctor, played by Peter Capaldi. What was it about Peter that you thought was right for the role?

He's one of the best actors in the country and is very beloved. I was at the BAFTAs shortly before we were contemplating Peter and heard the cheer he got from the audience. Nobody has a bad thing to say about him and that's not a minor issue when it comes to casting a Doctor. They've got to be lovely. And he's a huge fan of Doctor Who. So we asked and he was incredibly excited to come and audition. We didn't tell him that he was the only person auditioning because that would be oddly pressuring.

Did you deliberately aim to cast an older Doctor?

It wasn't the reason I cast Peter but I do think if we'd cast another Doctor as young as Matt - because Matt's been so good at being The Young Doctor - I'm not sure what another one would have done. They'd have to have either been deliberately different or just repeat him.

How did you find the readthrough? Was it emotional?

It was a very emotional read-through. Just going through the process of saying goodbye was difficult. The script itself is very emotional, but also joyous. But saying those words and saying goodbye was never going to be easy.

Is there a sense from the start of the episode that we're moving towards a regeneration?

It's very much an adventure, but it goes off on a different track. People watching will know that it's Matt's last episode, but it doesn't loom over from the start.

We were introduced to Clara as the impossible girl last year. Are we going to find out more about her family background?

Absolutely. I think there had to be a sense of mystery last year to make the plot work. What's really interesting is that it does feel like we're starting again and we get to see her home life as well as her life with the Doctor.

From a few pictures that have been released there are some of you cooking Christmas dinner. Have you ever done that in real life?

My mum does the Christmas cooking. It couldn't be any other way.

Are you looking forward to filming next year with Peter?

It will be a different show next year. We have a bit of a gap before we start filming the new series, so I have time to get my head around it all. Me and Peter will get together before Christmas to start rehearsing and the scripts will start coming in. I think when I came in there was just a week off in production where Arthur and Karen left, so that would have been a strange shift.

This episode is very much about Matt and the 11th Doctor and Clara and the 11th Doctor and their last adventure together. I have no idea where we're going to next series!

Where will you be watching the special this year? Will you be at home?

Yeah, I think so. Last year the whole family got a cottage together where mum still did the Christmas cooking. We'll definitely all be together.

What was your reaction when you heard the next Doctor was Peter Capaldi?

It was kind of that moment "Of course, makes sense." It's funny as I don't think he was one of the names that was originally being speculated about and it wasn't until the week before that his name came up. He's going to be so different to Matt and take the show in an interesting direction.

When were you told?

Matt and I were told together during the royal visit to Roath Lock studios. We could tell something was going on and we managed to pin the producers down and get it out of them!

How hard was it to keep the secret?

I think I've learnt my lesson in that the best thing to do is not to tell anybody and then you don't have your own paranoia that you've let something out.

What will you miss about working with Matt?

Everything! When you're reading a scene with him he can turn anything on its head. He's so inventive, clever and very funny. There's just so much that I'll miss about him.

What drew you to the role of Tasha Lem?

So you're sitting in your dressing room on an ordinary day (well, ordinary for an actress) playing a normal person who makes breakfast and loses her purse sometimes. Then along comes a call to play a galactic nun and whizz around the universe with Matt Smith. Now what girl wouldn't be drawn to that?

Can you tell us anything about how we first meet your character?

The Doctor goes to see Tasha as they are old friends. He needs her help and knows he will find it with her, as although she is fearsomely powerful she is loyal to her friend.

What can you tell us about your costume and make-up? Did you work closely with the costume designer?

The designer Howard Burden knows the Doctor Who world inside out and had an image of Tasha that I loved from the start, so there was very little need for me to suggest anything. I just climbed into it really. Emma Cowen created a look that was a little spooky, referencing "Blade Runner" a little. We both wanted her to look as she is described in the writing: imposing, human, but with a touch of alien.

In this special a whole host of the Doctor's most famous enemies come together, including Cybermen, The Silence, Weeping Angels and Daleks - how did you find filming opposite these monsters? Have you had to do anything similar for previous roles?

Most of my acting life I have played ordinary women so obviously monsters haven't factored much. However, I did love getting to face off with one of this lot in "The Time Of The Doctor".

How did you find working with departing Doctor Matt Smith?

Joyous. I had heard through friends that he was a nice guy - and he is - but what struck me most is how enthusiastic and engaged he was in his role, in every single scene we played. It so often happens that someone playing a character for years sits back a bit. He didn't. He was as full of energy and inventiveness as someone on their first day and it was truly good to be around.

Did you meet the next Doctor Peter Capaldi?

Yes. Peter came to set for the first time and it was good to see him again as we had briefly played boyfriend and girlfriend years ago and he was hilarious. He has an air about him, that man. Can't wait to see what he will do with his Doctor.

How was it filming in Cardiff?

Would it be obvious to say rainy? It was a place I had never been to, I didn't know anyone and was staying in a hotel which can be a bit glum. However, the thing I discovered about Cardiff is that it is the friendliest place I have ever been. Ever. The Capital of Friendly. I could live there now I think . . . but I would buy a new raincoat.

And finally, where will you be watching the Christmas special?

My mum and brothers are insisting on watching it on Christmas Day, but I find it excruciating to watch things I'm in with other people, so I will go for a pint with a friend.