"Doctor Who" casting: Steven Moffat says, "You know it when you see it" Matt Smith, left, Jenna Coleman and Steven Moffat at the "Doctor Who" panel presentation at Comic-Con International in San Diego on July 21, 2013. Kevin Winter / Getty Images "Doctor Who" lead writer and executive producer Steven Moffat excels at keeping secrets. Case in point: The identity of the next actor to portray the Doctor, the perpetually regenerating time traveler hero in the long-running BBC show. When Matt Smith, who plays the current incarnation of the Doctor, announced in June that he would be leaving the sci-fi adventure series at the end of this year, Moffat began fending off questions about who would replace the "Doctor Who" star. "I have lied my arse off for months," Moffat admitted during the show's Comic-Con International presentation in San Diego last month. Now, with the casting announcement slated for Sunday, the only thing fans can expect is the unexpected. "You want people to be shocked when the name comes out," Moffat said, sipping a cup of coffee in a San Diego hotel lounge. "You want people to go, "No, that would never work!" They did that with Matt, when they announced Matt. It was, "He's far too young, and he doesn't look right. He's got silly hair. You want people to go, "Oh, what have they done?" And they always get wrenched out of their comfort zone, and then they find the Doctor again. And there's such a range of what the Doctor can be. Jenna Coleman, left, and Matt Smith in an episode of "Doctor Who." Moffat, who was visiting the U.S. to bring "Doctor Who" and his other show "Sherlock" to Comic-Con, spoke about the challenges posed by the impending transition between Matt Smith and the next Doctor. The new actor will star opposite Jenna Coleman's character Clara, the Doctor's traveling companion, as well as several other characters who already have been introduced. With a familiar cast of characters and an established writing team, Moffat said, the new Doctor will be part of a "substantially different" landscape than the one outgoing star Smith faced when he landed the role. Smith arrived in 2010, at the end of Russell T Davies" run as executive producer and lead writer. Moffat would helm the long-running science-fiction adventure series, but the show's beloved star David Tennant was leaving, along with the executives and a slate of popular costars, including Catherine Tate and Billie Piper. What Moffat and Smith faced was, in many ways, a blank canvas. I can admit this now, all these years later. It wasn't an ideal situation to come into," Moffat said. "That was terrifying, because there was a real chance that people would look at it and say, "That's not even the same show," because in no physical sense was it." Moffat recalled devising Smith's debut in an episode titled "The Eleventh Hour" that would introduce the new Doctor, his new traveling companion Amy Pond (Karen Gillan) and a new design for his time machine and spaceship, the TARDIS. While devoted fans on both sides of the Atlantic mourned Tennant's departure, Smith's new bow-tie-wearing Doctor worked his own brand of quirky magic, enchanting viewers with a premiere that was part superhero story, part fairy tale. Matt Smith debuts as the 11th Doctor in the "Doctor Who" episode "The Eleventh Hour." "We really had to have such a charm offensive in "The Eleventh Hour" to sell the idea both that you like this guy who's taken David Tennant's place, and that somehow the same man is looking out of those eyes," Moffat said. We plotted that course so carefully. .. Whereas now, we've got a little cast around him, and the same TARDIS and all that, so the new Doctor can be launched in circumstances that you can challenge the audience a little bit with what the new Doctor is like. The new Doctor will be revealed at 11 a.m. PDT on Sunday, during a live TV presentation titled "Doctor Who Live: The Next Doctor" on BBC America and BBC One. The half-hour program will feature interviews with Smith, Moffat and previous Doctors and companions, in addition to introducing the show's new lead. Speculation about the casting abounds among fans, many of whom campaigned in online forums and fan groups for a woman or a person of color to portray the Time Lord's next incarnation. Several actors, including Peter Capaldi, Rory Kinnear and Ben Daniels have been cited as possible choices. For Moffat, the casting decision was more feeling than science. "You know it when you see it, and that's it," Moffat said. Matt was overwhelmingly right, for the reasons that everyone now knows. He is like an old man trapped in a young man's body. He was irresistible, the hipster boffin. The other thing is, essentially, at root, it's a star part. Someone who is a star or will be a star is going to play that part. Someone you can't stop looking at. New Doctor will be announced in live show