Major changes are under way for Doctor Who for the 50th anniversary of the classic U.K. sci-fi series, as Season 7 ended with a mysterious dark Doctor, and another actor will carry on the Time Lord's legacy before the year is out.

At Comic-Con International in San Diego, BBC America hosted a panel celebrating the series, from its origins in 1963 through to the present. On hand were stars Matt Smith and Jenna Coleman, executive producer and lead writer Steven Moffat, writer and executive producer Mark Gatiss, producer Marcus Wilson, and David Bradley, who plays the First Doctor William Hartnell in the Adventure in Space and Time special, which tells the story of the show's origin.

While the crowd waited for the actors to take the stage, a significant number raised sonic screwdrivers in salute. The panel began with a clip show, before moderator Craig Ferguson arrived at the podium, saying, "Hello, sweeties!"

He joked that, once the Q&A begam, "if the right question is asked, the Silence will fall." Then, "the people waiting for the Sons of Anarchy panel are very confused."

Ferguson noted that the Moffat era has been marked by considerable loss, citing as just one example the deaths of Amy and Rory. "They died in their 80s, how much more merciful could I be?" Moffat joked.

Talking about his history with the character before taking over in 2010 from Russell T. Davies, Moffat said he's always been a fan, and "I still love it, even though it ruins every day of my year."

Gatiss said his Adventure in Space and Time special celebrates "an unlikely group of people [coming] together in unlikely circumstances to put together the show we still enjoy today."

"Mark remembers John Pertwee as his first Doctor," Bradley said, "but I'm a few weeks older than him, so I remember Hartnell. It was every Sunday evening, what you did. And I wasn't in my teens, I was in my 20s. The pub will have to wait."

Ferguson remarked that Bradley's turn as Hartnell is a far cry from his role as Walder Frey on Game of Thrones. "A lot of people hate you," Ferguson said.

"For some reason, the wedding invitations have dried up," Bradley quipped.

The audience was then treated to footage from An Adventure in Space and Time showing BBC executives trying to recruit Hartnell to the role. They pitch the series as "C.S. Lewis meets H.G. Wells meets Father Christmas. That's the Doctor." To which Hartnell replies, "Doctor Who?"

Asked whether he plots the long arc of Doctor Who makes things up as he goes, Moffat replied, "It's a lot of it improvisation, but a lot of it we've been setting up for a very long while." He said, however, that there is value in flexibility. "The newest idea you've had is always the most exciting.”

On the subject of this year's Christmas special, which will be Smith's swansong, the actor said, "I think we'll try to make the Christmas special one of the best, most inventive, all of those things -- I'm determined to make it a real belter."

Ferguson next asked about the chemistry between the Doctor and his companions, which has varied from Doctor to Doctor and through the several humans who have shared the TARDIS. "The Doctor is the same man, but each actor plays it coming from a different emotional background," Smith said. "Dave [Tennant] was sort of more swashbuckling with the ladies, whereas mine is often sort of flummoxed. River is the ultimate alien to him."

"I think he's asexual, really," Coleman said.

An exclusive clip revealed several scenes from the special, what appears to be the infamous Time War that took place in an unseen era between the Eighth Doctor's made-for-TV movie and Ninth Doctor's debut in the 2005 relaunch. Between action sequences, Smith's and Tennant's Doctors can be seen comparing sonic screwdrivers and trying out each other's fashion; they also have a scene in the TARDIS together, alongside John Hurt's Dark Doctor.

After the clip, the floor was opened to fan questions.

Asked about their favorite moments from the previous season, Smith and Coleman cited Clara's scene with the eleven Doctors, with Coleman also enjoying running up the cloud staircase to the TARDIS.

Gatiss said his favorite moment was "getting Dame Diana Rigg on my second episode, and the moment when she pulled back this big Victorian dress to reveal a prehistoric leech is clutching her breast — that's something I never thought we'd see on British television."

A fan asked about more episodes set in America. "I want to go on record as wanting to film the Christmas special in America," Smith said. "Can we afford it?" Wilson said he'd love to go back if they had the story, but "the American crew didn't really know the show, and they said, 'Oh, you'll have no trouble getting around New York.'" But of course they were recognized and swamped by fans.

Regarding the spacefaring episodes and those taking place in the past, Moffat joked that he usually gets other writers to tacl;e the "historicals" because "it involves research."

He said what he loves about the Doctor is, "he always in the moment; he never looks back because he'd never stop looking." He added that "he's every age at once," from a child to a teenager to "a grumpy old man."

Despite rumors, Moffat protested that the new Doctor has not yet been cast.

Asked why John Barrowman's Captain Jack Harkness isn't in the anniversary special, Moffat said, "You can't put everybody in the 50th, and he's been gone a very long time." Then, "and how do you know what is and is not in the anniversary? I've been lying my arse off for months — you know nothing!" While this was by no means confirmation Barrowman would in fact appear, he added that "it's all based on story," and Captain Jack could return if the right story came along.

Smith talked about his adventures at Comic-Con: In addition to walking the convention floor as Bart Simpson, he and Coleman also had some fun while out and about in San Diego. "We have a game where we're driving around and if we see a Doctor and a Clara, we wound down the windows and say, 'Hey, Doctor!' How's it going!!'" Smith said. "We did that with one guy, we wound down the window and said, 'Hey, Doctor!' Great costume, how's it going?' And he was like, 'Eh?' [grumpily] The Clara behind him tapped his shoulder, and then he was like, 'Oh no!'"

Asked what other character they'd like to play, Smith offered the Master while Coleman said Strax. "You're not tall enough," Smith said.

"I'm not going to lie, I'd want to be the Doctor," Moffat said. "I could never be the Doctor because I'm rubbish at acting."

Ferguson said the audience in America seems younger than that in Britain, but Moffat noted in the United States "there isn't a huge audience of children." He noted that American viewers tend to be in their 20s, while in the United Kingdom viewers often lose interest in their 20s and then perhaps come back later.

Ferguson asked the audience how many watched Who when it airs, and how many "watch it illegally at a later time." That option received loud applause, to which Ferguson replied, "I think that's a wakeup call to everybody in the TV business."

A fan asked about what sort of alien monster the stars would like to create for the show. "Mine would have to be big," Smith said. "I'm picturing lots of arms," Coleman offered.

"I like the ones that chase you. No, I actually like the psychological ones," Smith added, before admitting that monster design wasn't really his forte. "We just turn up and get our lines."

"The Whisper Men were my favorite from last season," Coleman said.

Continuing the discussion of monsters, Ferguson was a bit perplexed at the idea of Weeping Angels action figures. Moffat remarked that crafting a story for the angels is more difficult than one might think. 'Those are hell to write, because you have to write chase scenes with a thing that don't move," Moffat said.

He then recounted a story of Smith playing the hero at "Crashing the Elysium," which he described as "an immersive theater experience where you're taken through a crashed ship and are attacked by Weeping Angels." "Matt was going to make a personal appearance, but before he did that he decided he wanted to go around it and see what it was like," Moffat said. "So he put his hoodie on and snuck in at the back. He saw one particularly frightened little child, and Matt walked over to her and told her it's not real. And she looked up to see the Doctor!"