The Christmas story sees the return of Bernard Cribbins and John Simm
By Tim Masters
Entertainment correspondent, BBC News
Doctor Who chief writer Russell T Davies has said he is very proud of the final episodes that mark the end of David Tennant's reign as the Time Lord.
Davies, who masterminded the sci-fi show's return in 2005, also said he was no longer sad to be leaving as the show's producer.
"It's the end for us, but not the end for Doctor Who," he told the BBC.
He was speaking before a preview screening of episode one of Tennant's final story, The End of Time.
"All the sad bits I did when I was writing it, so I got that bit out of my system then. So I could then stand back and laugh while everyone else was blubbing their eyes out," Davies said.
"I'm very proud of these two episodes," he said. "Now I simply feel happy to be honest, really happy with what we made.
The story pitches The Doctor against his nemesis The Master (John Simm)
"We've got one or two surprises in store before the episode on New Year's Day - and it might not be quite what you expect."
Tennant is being replaced by actor Matt Smith, who will appear as the 11th Doctor next year.
Davies said he was keeping in touch with Steven Moffat, who has taken over as Doctor Who's chief show-runner.
"He does find time to e-mail me every so often saying it's all a nightmare, and he's never been happier in his life," Davies said.
"I've read some of the scripts - they are beyond brilliant - I can't tell you what thrills and darkness and comedy you've got to come."
The Christmas Day story on BBC One features the return of Bernard Cribbins, Catherine Tate and John Simm as The Master, the Doctor's evil nemesis.
"He's even more insane than before," Simm said. "There was no limit in the script to how insane he should be. It was a lot of fun to do."
He added: "I don't think it's that scary - I don't think we're allowed to scare children on Christmas Day. But I'm not an eight-year-old kid!"
The Life on Mars star said it had been a "real honour" to be asked to come back for the 10th Doctor's final story.
His words were echoed by comedienne Catherine Tate, who returns as Donna Noble.
David Tennant on leaving Doctor Who behind
"I feel so delighted that I was even part of a bit of it, let alone to have been a companion and also in David's last two episodes - that's a real honour."
Tate said her lack of Doctor Who expertise had been a balance to Tennant's encyclopaedic knowledge on set.
"He knows a lot and I know nothing, and we meet in the middle happily," she said.
"I turn up on the day and say: 'I don't actually understand what's happening in this scene,' and he'll go: 'well, it's a meta-crisis and there's been some sort of transformation,' and it means nothing to me.
"But at least I've learnt my lines."
Tate added that she was keen to attend a Doctor Who convention for the first time.
"I hear they're very exciting and so I would love to go to one. I guess it's exciting... and a little bit scary. Let's face it, I'm going to be the person in the room that knows the least."
The End of Time: Part 1 is on BBC One at 1800 GMT with Part 2 on New Year's Day at 1840 GMT.
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