The birth of a true Christmas special

And so a Christmas Day truce has been brokered. The controller of BBC1, Danny Cohen, and his opposite number at ITV, Peter Fincham, have looked long and hard at the Christmas-night schedules and blinked, deciding that their respective big-hitters, Call the Midwife and Downton Abbey should be kept apart. Their dramas now segue neatly, thus saving the nation domestic strife at an already combustible time of year.

But none of this concerns Miranda Hart, as she huddles beneath a blanket in the freezing former Catholic seminary in Mill Hill, north London that doubles for Nonnatus House, the convent at the big but not excessively squidgy heart of Call the Midwife. "I'm not competitive like that", says Hart. "It's for executives to think like that – I'm just thrilled to do the work."

In fact, Hart will be appearing in not just one, but two of the biggest attractions this festive season, the new series of her sitcom Miranda following on Boxing Day. "When I was younger, BBC1 at Christmas was massive, so to be on it is quite surreal", she says, "But if you think about it you can get confused and nervous on set. We were actually filming in this room and I suddenly thought 'God, this is going out on Christmas Day' and someone shouted 'action', and I was 'Oh no!'"

"This room" is the currently unheated hall that doubles as the community health centre, and it's been dressed with neo-natal equipment from the period. Based on the best-selling memoirs of East End midwife Jennifer Worth, and skilfully adapted by Cranford screenwriter Heidi Thomas, the first series became the BBC's biggest drama hit in over 10 years, overtaking Downton Abbey's audience share.

Call the Midwife has followed the saga of Edwardian aristocrats across the Atlantic, PBS in America having joined the BBC as co-partners for this second series. "We met the cast of Downton Abbey on the plane over to promote the show in America and had a drink with them", says Jessica Raine, who plays Jenny Lee – the Jennifer Worth character through whose novice eyes we see the succession of breach births, unwanted pregnancies and premature babies. "From us there is no competition with Downton", she says. "In terms of American success, it has led the way for British drama."

But what is this elusive success based on? The common wisdom is that Call the Midwife, like Downton Abbey, provides escape from hard economic times, but Raine thinks that is only part of the answer. "My fear always is I don't want it to be chocolate box and it never is", she says. "They're quite dark some of these stories. It doesn't patronise the audience."

Perhaps the biggest unknown for its creators was how wide would be the appeal. "I didn't think men would watch it and I was so wrong", says producer Hugh Warren. "A lot of young men are involved in childbirth now. Also I thought the audience would be predominantly older.

"The babies are a big factor of course. We're so hardwired to respond to newborn babies – even the crew. We rehearsed one scene and I turned around and people were literally crying… I've never experienced that before. ."

Despite some graphic birth scenes that employ a mix of real newborns (often twins because they are smaller and look younger), prosthetic infants and fake blood, children are among the show's biggest fans. Helen Georg, who plays midwife Trixie, suggests that "families are using it as a form of sex education".

Another plank in the show's success is the casting – a mixture of appealing fresh faces like George and Raine, and veterans such as Jenny Agutter, Pam Ferris and Judy Parfitt. Perhaps the biggest coup was one suggested by Worth herself before she died last year, while the casting was still in progress. Worth wrote to Miranda Hart to announce that she would be perfect to play Camilla "Chummy" Browne, the physically awkward posh midwife.

"The bizarre thing was that when we were first casting, a number of actors, having read the script said 'That Chummy… that's Miranda Hart, isn't it?'", says Warren. "And we had to keep our mouths shut because the deal hadn't been closed. I was worried that the character Miranda from her sitcom would swamp the show, but Miranda is brilliant… she's a brilliant actor."

Still huddled under her blanket, Hart admits that it was a worry for her too. "Some people felt it wasn't a stretch but it felt to me that it was very, very different", she says. "The obvious thing to do would have been to do the Joyce Grenfell thing… and I found it very difficult not to do the 'jolly hockey sticks' thing… She's always saying 'one's'… So I deliberately did the opposite and the directors were great when filming it – they'd say 'No, you can't fall over or smash into things.'

"I've had more fun doing this than I did making the sitcom," she adds. Not that Hart will be enjoying Call the Midwife or Miranda. "I couldn't sit down and watch it with my family," she says. "Marvellous… look at me… look everyone."

Call the Midwife is on Christmas Day at 7.30pm on BBC1; Downton Abbey is at 8.45pm on ITV1; Miranda is on Boxing Day at 9pm on BBC1