'What happens next isn't great'

Rome actress Lindsay Duncan joins David Tennant in The Waters Of Mars

One of the final episodes of Doctor Who featuring David Tennant as The Doctor airs on TV this weekend (15 November). Called The Waters Of Mars, the story is set in the year 2059 and sees the doctor joined by a new assistant, Adelaide Brooke, who's played by Lindsay Duncan. A two-part special to be broadcast over Christmas will be David Tennant's final scenes. He's spoken to Newsbeat about this 'explosive' new episode. ____________________________________________________

Does it feel weird to be promoting Doctor Who when it's kind of in the past for you?

It's not though really. This show gets so much attention and elicits so much enthusiasm from so many people that it lives with you every day really, even when you're off doing other things as I have been. Doctor Who is everywhere and you're constantly reminded of it and that's lovely.

The writers say they've really upped their game for your final episodes. How grand is it and how dramatic and explosive is it?

I feel like I've spent four years going, 'This is the biggest episode we've ever done'. And whilst that has tended to be true actually, we do keep managing to make bigger and bolder stuff and the special effects team do things they've never done before, and that's both the computer effects and also the practical effects.

We have bigger explosions in these episodes than we've ever had. We will do things to some of the standing sets that we've never been able to do before.

Because you can only tell the story of a character dying once, it allows you to go to places that you haven't quite been before and it allows you to tell types of stories that you can't do when it's an ongoing series and you have to reset to zero at the end of every episode.

It allows it to be bigger and more epic and sadder and wilder and the stakes are just that bit higher. So everything else cranks up alongside that I suppose.

How much input did you have in the final episodes? Were you able to say, 'I'd really like to try this' or 'I want Catherine Tate to come back'?

I've never done that to be honest because it's Russell and because Russell T Davies is sort of the best writer in television.

Because Russell's writing it it would be foolish of me to start telling him what to do. Maybe if he ran out of inspiration or something but wait until you see these final stories. They're just incredible.

What can you tell us about this next episode, The Waters Of Mars? You've got Lindsay Duncan as a co-star, who's not normally known for her action roles. What can you tell us about her part?

Lindsay Duncan plays Captain Adelaide Brooke, who it turns out is going to be terribly important in the earth's future.

Former Bad Girls actress Sharon Duncan Brewster plays Maggie Cain

When the doctor meets her, to him, it's the same as meeting Shakespeare or the same as meeting Agatha Christie, these people who for him, because our future is his past, are as big a historical figure as Shakespeare was and he gets just as excited about meeting her.

So you needed somebody who brings that kind of gravity, and indeed elegance, but also strength and power and we couldn't have done it without an actress like Lindsay.

And when you see the episode you'll see that she just embodies this extraordinary character and she's the alpha male, between her and the doctor, she's the alpha male in the room.

When the doctor lands and realises that that's who he's bumped into, he knows what's going to happen next, which for the doctor is a very hard thing to come to terms with because what happens next isn't great.

The Waters of Mars is on BBC One at 7pm on Sunday 15 November. A two-part special, The End Of Time, will be broadcast over Christmas.