It's the first scene to be shot after lunch and Miranda Hart's Call the Midwife character, Camilla "Chummy" Noakes, has to lift her baby son Freddie from his cot. It's a sequence that is supposed to convey Chummy's re-connecting with her offspring after returning to her nursing job, except that 14-month-old George (one of the four sets of twins to have played Freddie since the current series began filming way back in the summer), is not ready yet for such a touching scene. In fact, he's bawling his head off.

According to his mother watching nearby, George had been up all night teething – but by dint of jigging and whispering soothing noises in George's ear, Hart soon has the toddler mollified. "Motherhood is lifting a heavy baby," she says afterwards. "It's new muscles. He was very well behaved, actually.

"Chummy wasn't loved emotionally and physically as a young child and so she's pouring out that need on to her son – in a healthy way, I think," Hart says of her character's journey in the forthcoming new series. "But she's also got that real yearning to get back to her vocation. So the beginning of the series is quite an interesting place, which is motherhood and juggling the prospect of going back to work."

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The last time I visited Hart on the set of Call the Midwife, she had been huddled beneath a blanket in a vast deep-freeze of a hall that doubled as the Poplar community centre. In the intervening 12 months the former Jesuit missionary college in Mill Hill, north London, that was used as the set of Nonnatus House, the fictional home of Call the Midwife's 1950s health-workers, has been sold for re-development as a luxury housing estate. Production was forced to move to its current home, a somewhat dilapidated Gothic mansion on the western fringes of the M25 that used to house the officer's mess at a local RAF base.

Hart expresses surprise at the spate of news stories when they had to switch production bases in the summer. "But then when you're in a show, you forget there are fans of it… people are interested," she says. "I'm fanatical about other shows and you suddenly remember 'of course people love it'. You just can't get fanatical about something you're in."

Call the Midwife is not just big in Britain – beating even the mighty Downton Abbey in the ratings – but a hit in America (where it screens, like Downton, on PBS) and the rest of the English-speaking world (Spanish-speaking countries have also taken to it, says writer and executive producer Heidi Thomas, "perhaps because I had a Spanish character in the very first episode".) Hart's scene-stealing performance as Chummy is consistently singled out for praise by the US critics.

"I'm so thrilled she (Hart) keeps coming back again and again because the thing about Chummy in the books is that she jumps off the page but there isn't a great deal of depth to her," says Thomas, who adapted Cranford and the re-booted Upstairs Downstairs, before Call the Midwife. "She's an actress of some depth and complexity and I find myself writing for that part of her rather than the comedic part.

"She's very intuitive and intellectually clever but she also has this light spirit. Quite recently I was on set and it was a tough day, and Miranda was singing, dancing and laughing – she keeps the energy up between takes. Apparently yesterday she had all the cast on the steps singing "2 Become 1" by the Spice Girls, with Miranda very much as the ringleader."

Shape Created with Sketch. Call the Midwife Christmas special Show all 15 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. Call the Midwife Christmas special 1/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee), Bryony Hannah (Cynthia Miller), Helen George (Trixie) 2/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee) at the Christmas market 3/15 Call the Midwife Christmas special Miranda Hart (Chummy Noakes) 4/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee) 5/15 Call the Midwife Christmas special Max Macmillan (Timothy Turner), Stephen McGann (Dr Turner) 6/15 Call the Midwife Christmas special Ben Caplan (PC Peter Noakes), Raymond Coulthard (Captain Goodacre), Cliff Parisi (Fred) 7/15 Call the Midwife Christmas special Judy Parfitt (Sister Monica Joan), Jenny Agutter (Sister Julienne) , Pam Ferris (Sister Evangelina) 8/15 Call the Midwife Christmas special Sandi Toksvig (Sister Gibbs) 9/15 Call the Midwife Christmas special Cubs at the Christmas market 10/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee), Helen George (Trixie) 11/15 Call the Midwife Christmas special Cubs singing at the evacuation centre 12/15 Call the Midwife Christmas special Behind the scenes image: Helen George (Trixie Foster), Leo Starr (Alec Jesmond), Jessica Raine (Jenny Lee), Miranda Hart (Chummy), Bryony Hannah (Cynthia Miller) 13/15 Call the Midwife Christmas special Playing with the cubs Bryony Hannah (Cynthia Miller) 14/15 Call the Midwife Christmas special With baby Freddie Ben Caplan (PC Peter Noakes), Miranda Hart (Chummy) 15/15 Call the Midwife Christmas special Holding Freddie at the cubs' Christmas party Jake Bailey (Jack) 1/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee), Bryony Hannah (Cynthia Miller), Helen George (Trixie) 2/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee) at the Christmas market 3/15 Call the Midwife Christmas special Miranda Hart (Chummy Noakes) 4/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee) 5/15 Call the Midwife Christmas special Max Macmillan (Timothy Turner), Stephen McGann (Dr Turner) 6/15 Call the Midwife Christmas special Ben Caplan (PC Peter Noakes), Raymond Coulthard (Captain Goodacre), Cliff Parisi (Fred) 7/15 Call the Midwife Christmas special Judy Parfitt (Sister Monica Joan), Jenny Agutter (Sister Julienne) , Pam Ferris (Sister Evangelina) 8/15 Call the Midwife Christmas special Sandi Toksvig (Sister Gibbs) 9/15 Call the Midwife Christmas special Cubs at the Christmas market 10/15 Call the Midwife Christmas special Jessica Raine (Jenny Lee), Helen George (Trixie) 11/15 Call the Midwife Christmas special Cubs singing at the evacuation centre 12/15 Call the Midwife Christmas special Behind the scenes image: Helen George (Trixie Foster), Leo Starr (Alec Jesmond), Jessica Raine (Jenny Lee), Miranda Hart (Chummy), Bryony Hannah (Cynthia Miller) 13/15 Call the Midwife Christmas special Playing with the cubs Bryony Hannah (Cynthia Miller) 14/15 Call the Midwife Christmas special With baby Freddie Ben Caplan (PC Peter Noakes), Miranda Hart (Chummy) 15/15 Call the Midwife Christmas special Holding Freddie at the cubs' Christmas party Jake Bailey (Jack)

"Yeah, absolutely," she says now. "Particularly with a hit show and you know that people are longing for it to come back. It's great when you're not shouldering that burden and, more to the point, somebody else is writing it. It's lovely just to be the actress."

So when can we expect a new series of Miranda? "I'm doing my tour first so this year I've been writing that and doing Call the Midwife," she says. "I think once you do something different and step away from a big project like the sitcom then I get a sense of what I want to do creatively with it and how much I miss it and all that. So hopefully we'll do a couple of specials next year and then go from there."

Hart's arena tour, her first set of stand-up comedy engagements in seven years, begins at the end of February and continues until mid-April, before resuming briefly again next October. Will she be changing the nature of her material to suit the sorts of large venues that she couldn't have hoped to fill in her pre-Miranda stand-up days? "Whatever venue I was in I would to make it as theatrical as possible… as big a show as possible for people," she says. "It's just slightly bizarre that stand-up comedians can play Wembley but hopefully it will be quite an experience.

"I started doing some warm-up gigs in the summer and actually within three or four gigs I felt reminded of how I am on stage and it was nice to have a warmth from the audience because they know me from the sitcom. So I then felt confident quite quickly which was nice and a bit of a relief."

Meanwhile, Hart jokes about potentially being in series nine of Call the Midwife, when the action has moved on to the late 1960s, and the possibility of Chummy starting to experiment with drugs. "She's probably not the most progressive person," she says. "There's something about Chummy in the Sixties which feels hilarious. Perhaps it should be a spinoff sitcom: 'Chummy'!"

For the time being, however, we're still in the late Fifties. What does Hart believe to be the essential appeal of the show? "It's so subjective what you take away from it," she says. "For me, it's a sort of simpler life… real community and the family and friendship that sort of doesn't always really exist now. We're much more isolated and communication is virtual." Hart herself has well over a million followers on Twitter, to whom, in October, she broadcast an appeal after her laptop, containing unspecified new material, was stolen from her home in west London (she never did get it back). "I hate Twitter," she says. "I am on it and I do occasionally say 'Do you want to buy tickets for my tour?' and then I feel a bit embarrassed."

Interestingly, in a year when President Obama has struggled to pass his health-care legislation, critics in the US have flagged up the political aspects of the show, with the LA Times calling it "a timely Valentine to socialised medicine". Hart's younger co-star, Jessica Raine, reckons Call the Midwife is also a feminist drama. "I am very proud to say that it is," she says. "It's rare… too rare… that you see a big series with so many women characters, women of all ages as well."

Pam Ferris, who took the role of Sister Evangelina "thinking it would be a nice little job… ha-ha-ha", agrees with Raine. "I love it on set sometimes when we're sitting around that table we can have eight women," she says. "And we have great conversations, which wouldn't happen if there was mixed gender… there would be games and chat and silliness. We get into quite serious conversations."

Hart is clear about what she sees as the appeal of Chummy – an upper-class character in a drama that consistently champions the lower orders. "Because she wasn't particularly loved when she was younger she's really empathetic," says Hart. "She's been through stuff herself, which gives her the perfect emotional intelligence for her job. She may have been a bit clumsy to start with, but she has the best bedside manner of any of the nurses. I just love the fact that when people have been through rubbish times in their life they are always the ones who are the most compassionate.

"I love Heidi's writing… I loved Cranford and I think she's an absolute genius. But you don't know she might suddenly decide to kill Chummy… you don't know what she's thinking. But I would hate the thought of never playing Chummy again."

Call the Midwife is broadcast on Christmas Day at 6.15pm on BBC1. The new series starts in January