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Tonight Doctor Who fans get to gorge on their annual Christmas fix -- a full-length special episode the series has produced every year for the holiday since 2005. This time, however, there's some extra spice in the form of a new regular cast member: Jenna-Louise Coleman debuts in "The Snowmen" as the Doctor's next companion.

Except it's not her debut. Coleman actually made her first appearance in the series premiere back in September. Actually, make that surprise appearance. In preseason interviews, Doctor Who's producers had explicitly told fans they'd have to wait until Christmas before they'd see Coleman in the show.

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But there she was, fighting Daleks and making souffl s, way ahead of schedule. This was unheard of for the series, which has seen major plotlines leak online -- usually months before broadcast -- several times over the past few years. The show had gotten to the point where it would simply announce any major developments far in advance in order to get ahead of the spoiler hunters.

Yet somehow the show's producers kept Coleman's early debut a secret -- a feat made even more challenging since there were several preview screenings of the episode, each attended by hundreds of rabid fans, all carrying smartphones. How did Doctor Who keep every single one of them from tweeting about it?

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"I asked. That's it," says Steven Moffat, Doctor Who's current showrunner. "I don't think anyone thought it would work. I certainly didn't. At the London premiere, I just stood up and said, 'Please, nobody, no fan, no newspaper -- nobody at all -- mention that she's in it. And to my surprise it worked."

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Moffat says the idea of misleading the audience about when Coleman would debut "grew" as he was writing the current series. But it almost didn't happen since others at the BBC wanted to get ahead of the news and announce her presence at the first preview screening. Moffat, however, was convinced (rightly, it turns out) that he could persuade the fans and journalists in attendance to guard the secret.

"They tried to talk me out of it at the last minute," he says. "And it did involve a lot of charming journalists and saying 'Please don't...' It was the polite embargo, really. We couldn't really embargo it. And I was always clear, 'There is no punishment here. You don't get blacklisted -- I'm just asking, and the show will be better if you keep this secret.' And they did."

But did really not a single person on fire off a quick tweet about Coleman being on the show? It appears so. Although Twitter doesn't offer a way to search tweets within a specific date range, searching the Twitter domain on Google during the month of August (the series premiered on Sept. 1) for her name reveals just regular promotion for the show.

"You can get a long way just by asking politely," says Moffat. "Who knew that's all you had to do? What's remarkable about it is not one single person broke. And I really didn't think that was going to work, because if any website had broken it -- if any forum had broken it -- the press would have just leapt in. They would have felt no further need for restraint. But they didn't."

Now Coleman makes her "proper" debut in the Christmas special, but is she playing the same character as before (who was -- spoiler alert -- abruptly killed off), or someone different? Moffat's already told fans not to expect any great explanations under the tree. What's going on with Coleman's character (characters?) won't be fully revealed until the series returns in the New Year.

But who knows? Maybe that's another mislead.

Will you be watching Doctor Who tonight? Does the show still surprise you? Share your thoughts in the comments.

BONUS: Doctor Who Series 7 Premiere

Doctor Who Returns Matt Smith (The Doctor) and Karen Gillan (Amy Pond) attended a special screening of the premiere of Doctor Who Series 7 at New York City's Ziegfeld Theater. The episode, "Asylum of the Daleks," debuts on BBC America on Sept. 1.

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This story originally published on Mashable here.