Former Emmerdale actor Jenna-Louise Coleman is to join Doctor Who as Matt Smith's new companion – but viewers will have to wait until the show's Christmas special to meet her.

Coleman will take over from Karen Gillan, who has played Amy Pond in the show for the past two series, in what showrunner Steven Moffat described as "a very, very different way for the Doctor to meet his new friend".

The writer refused to even reveal the name of Coleman's character in the show. "Who she's playing, how the Doctor meets her, and even where he finds her are all part of one of the biggest mysteries the Time Lord ever encounters," Moffat said. "Even by the Doctor's standards, this isn't your usual boy meets girl."

Coleman, who is due to start work on the show next weekend, has had to keep the news of her new role secret. "A few members of my family knew but it's a huge relief today to be able to phone up my flatmate and tell them," she said.

"I've had a few conversations with Matt who's been amazing. He texted me this morning wishing me good luck and 'We'll have the best time ever,' I think were his words. So he's been very supportive and very excited."

Saying she was a fan of the show, the actor said that David Tennant and Billie Piper were probably the Doctor and companion she was most fond of – alongside Smith and Gillan, of course.

The seventh series of the BBC's revived Doctor Who will kick off with six episodes including the Christmas special airing this autumn, with a further eight episodes to follow in early 2013.

"Amy and Rory will leave in the fifth [episode] that goes out," said Moffat. "It will be a final encounter with the angels and not everybody gets out alive – and I mean it this time."

The show will celebrate its 50th anniversary next year, but the writer refused to be drawn on what plans he had in store.

It's still not certain that Matt Smith will remain in his role as the Doctor for an eighth series of the show. "We take it a year at a time and that's the only sensible way you can deal with it. It's a huge amount of time filming a series of Doctor Who and towards the end of that you have to consider if you're going to push onto another one. So the honest answer is I don't know," said Moffat.

"We'll keep him as long as we can is the absolute truth and he hasn't made up his mind yet."

Coleman won her part on the show after impressing with her chemistry with Smith. Moffat said the actor, who will appear in ITV1's big-budget Titanic on Sunday, and has previously appeared in Emmerdale and Waterloo Road, was the only person he'd met who could go faster than Smith.

"There was a moment I remember when they were auditioning together … and they both look frightened doing a Doctor Who scene, and it was an instant poster and I thought 'God that's brilliant'," Moffat said.

Coleman, who is 25 and from Blackpool, said that she hopes she is prepared for the huge interest that surrounds the show and the pressures that brings but is trying to focus on the job. "What I feel really comfortable with is the meetings that I've had with Matt and how much fun we had," she said.

"That is what I'm focusing on and why I can't wait to get on set, and read all the scripts and just start doing my job really," she said. "So I think I'm trying not to really look at those pressures and just be excited about the job in itself."

The actor found she'd got the job while she was in Marks & Spencer buying the ingredients for a salad. "I was in Marks & Spencers holding an avocado," she laughed. "I put my basket down and left Marks & Spencers. But I couldn't tell anyone so I went for a bit of a walk and tried to digest. It was really quite surreal."

Moffat, who is also co-creator of the BBC's hugely successful Sherlock, was tight-lipped about US broadcaster CBS's new take on Sherlock Holmes, Elementary, which stars Johnny-Lee Miller and Lucy Liu.

"They approached to reversion our show, we said no and they just decided to make one anyway. So I'll just leave you to speculate on what I think about that," he said.