Doctor Who masterclass @ Edinburgh TV Festival

I was lucky enough to be in the audience for the Doctor Who masterclass at the Edinburgh International Television Festival this morning. The big announcement was that Doctor Who in 2011 series will effectively be two series. Moffat said the next series would begin at Easter with a run of seven episodes. The seventh ep would feature an earth-shattering climax that means nothing will ever be the same for the Doctor, Amy or Rory.

"It's an enormous, game-changing cliffhanger," Moffat told delegates. "Doctor Who will return in Autumn 2011 for another six episodes. We're not splitting it, we're making two separate series. It means you will never be more than a few months from the next series. Tart that I am, it means I get twice the number of first nights, twice the number of finales. The cliffhanger will be a doozy, a big one, a belter."

Moffat said the decision was inspired by watching his first series as showrunner when it was being transmitted: "I thought this show needs a big event in the middle, a mid-series finale." For him the show is epic, like climbing a mountain so large you can't see beyond it. "We have to take it series by series. Each one fills the horizon."

Moffat said he'd never worked so hard in his life. "For these years [while I'm running Doctor Who] I am a lazy bastard stuck in the body of a workaholic." It was confirmed during the festival that Moffat's other show, an updated version of Sherlock [co-created by Mark Gatiss], would be returning for another trio of 90-minute stories. "Two enormous BBC hits," Moffat observed, "that's not too bad."

"The Doctor and Sherlock are similar in intellect, but the Doctor is more human than Sherlock. I like to think of the Doctor as an angel who aspires to humanity. Sherlock starts as an amoral, autistic bastard who will be redeemed by friendship, made human by a psychopath." Moffat said Benedict Cumberbatch was cast as Sherlock before casting for the 11th Doctor began, so Cumberbatch was never considered for Who.

Moffat believes he has the best job in the world: "It's the only job where I get referred to as a supremo and as a dark overlord." He said Matt Smith's audition was so good, the tape could have been shown to the public and they would have made the exact same choice. "We knew we couldn't improve on that. Matt's version of the Doctor - he's like a young man built by old men from memory."

Moffat first saw Karen Gillan via tape, and claimed it made her look wee and dumpy. When she turned up, the actress was "this flame-haired goddess. When she wears heels, she's terrifying. You feel like the sidecar on a motorbike next to her." Gillan was also at the masterclass. She choose Amy's mini-skirt costume. "That came from her being a kiss-o-gram. She isn't going to wear big, massive trousers."

Moffat laughed about an article complaining Amy Pond was too sexy to be a companion on Who. "One of his past companions wore a leather bikini!" He said anyone taking an episode to a stag party because of its supposed sexiness would be disappointed.

Moffat is jointly responsible for the creative vision, but doesn't want writers replicating him: "I say to the writers, don't imitate me. You have to behave as if the show's yours." Predecessor Russell T Davies sent Moffat long, enthusiastic emails after each new episode. Moffat does take a pass at scripts by other writers if required, but tries to give it back to them before the script gets locked.

He didn't dismiss the idea of the Doctor regenerating into a woman in the future, but wasn't worried the character is supposed to run out of incarnations at number 13. "If we got to the 13th Doctor dying, we would involve the emergency protocol of making something up." Moffat just does the stuff he wants to see. "With my level of emotional maturity, that's perfect for any three-year-olds watching," he quipped.

Gillan said the Weeping Angels were her favourite monster on the show. Moffat admitted being proud of them, but they do offer creative challenges: "It's hell to write chase scenes with inanimate objects."

A brief trailer for the Christmas episode was shown, with the tagline [Christmas] Time Can Be Rewritten. The trailer suggests this will be Who's version of A Christmas Carol by Charles Dickens, but that could be subterfuge. Moffat will be seeing a first cut of this year's Christmas special on Tuesday. Beyond that, he confirmed the next series will reveal the true identity of River Song.