Tennant said he was excited for new Doctor Matt Smith, saying: "I remember how exciting it was starting out on this kind of a journey and nerve-wracking. I'm jealous that he's going through that now - but it couldn't happen to a nicer chap. I'm looking forward to not knowing what happens next. I'm looking forward to being a viewer again."
Of the special, Tennant said: "The Doctor finds himself in a situation where he knows what the end is. It's all about whether he can un-knit the inevitable. With this particular Time Lord's life coming to an end, if he starts fiddling with the fundamentals of time and space, it might lead to his undoing."
At the end of the episode, viewers will glimpse some familiar faces who will appear in the Doctor's adventures to come, including Catherine Tate, who plays the Doctor's former companion Donna, and his arch-nemesis the Master, played by John Simm. He teased: "It's fitting and proper he should be there to see the Doctor off - if that's what happens."
Tennant said: "The Doctor now knows incontrovertibly that he's running from his own demise." He described shooting the final scenes as "emotional" but said that as they were filmed out of sequence, the situation did not get too weepy. The actor said the last line he said as the Doctor was: "'You two, with me, spit spot'... so it was robbed of any epic quality. It was very emotional saying cheerio. Filming the final scene was very, very sad. There's lots of scenes in the final, final story that are very sad and were very sad to play."
Tennant spoke about the big opportunities that arose in killing off the current tenth incarnation of the Doctor, to take the story "to places we've never quite been before". He said: "It allows it to be bigger and more epic and wilder... and sadder."
Tennant said he had originally felt under some pressure in taking on the role, saying: "It means so much to so many people. It meant so much to me." He said the bit he would miss the most was "when the new script comes. That was always such a thrill. It was always surprising." Asked if he felt he was in danger of becoming typecast, Tennant said: "I seem to have managed to do quite a variety of things while I've been doing Doctor Who. I haven't felt that it has been anything other than a positive." The star said he had his own sonic screwdriver which he kept locked up in a "secure location" in case his house was burgled.
On playing the role, he said: "You're not really expected to follow what went before, you're sort of expected to go your own way and mess it up a little bit the Doctor is different each time. You know James Bond is always James Bond and Tarzan is always Tarzan and Sherlock Holmes is always Sherlock Holmes but the Doctor is up to you, it's a blank sheet and you can scribble all over it, it's up to you."
He compared the job to being the United States president, saying: "You always get to be called the Doctor."
At the launch, Executive producer Russell T Davies confirmed The Waters of Mars will be dedicated to former producer Barry Letts. Davies paid tribute to his predecessor saying "He was one of the finest producers of Doctor Who and many programmes.. He used to do the Sunday afternoon classic serials and he actually produced the Jon Pertwee years. And then he cast Tom Baker. He cast Lis Sladen as well. He was an extraordinary figure in Doctor Who history and in pop culture – and he passed away, sadly. So, of course, we wanted to have that tribute to him on screen."
The launch has been covered by BBC News, The Guardian and BBC Newsbeat.
Journalist Ian Wylie has posted the full transcript of the press launch with Tennant and Davies on his blog.