Always the best friend: Sally Phillips on Christianity, comedians and the class system

Gerard Gilbert meets the perfectly-judged sidekick of Bridget Jones and Miranda

So you think that tickets for the London Olympics were over-subscribed? That was nothing on getting a seat to watch BBC1's Miranda being filmed. One week, according to Miranda Hart's co-star, Sally Phillips, who plays Miranda's friend, "monster idiot" and "language disease", Tilly, there were 16,000 applicants for just 300 places – and a large proportion of these were teenage girls.

"She [Miranda] is greeted when she goes to book signings like she's One Direction," says Phillips. "Teenage girls lying down on thef floor vomiting… it's really very extreme… and it is very good."

Good? How so? "Miranda's a wonderful model of girl friendship, I think," says Phillips. "The model must be a big relief if you're 15. She [Miranda] goes on a date and the way that it is plotted is that she meets the guy for two seconds and then unpacks it for four minutes with Sarah Hadland (who plays her joke-shop colleague, Stevie) and then you don't even see the date and she unpacks it again for another four minutes with Sarah Hadland. It's all about the relationship with her mates.

"Teenage girls are being exposed to pornography much earlier," continues Phillips, taking her theme in an unexpected direction. "Someone was telling me that at their child's school, the girls are saying that they are gay to get out of the sexual demands of the teenage boys. In that rather brutal environment, what a wonderful jukebox of joy that relationship model must be."

I was looking forward to meeting Phillips, having always liked her dimply screen persona and found her highly amusing, whether in Smack the Pony, or I'm Alan Partridge, or the Bridget Jones movies, but being told by the BBC PR two minutes before our interview that she wouldn't talk about her children – or more precisely, her seven-year-old son, Ollie, who was born with Down syndrome – or her faith (Phillips is a practising Christian) was discombobulating.

Not wanting to discuss Ollie is entirely understandable, of course, as she herself puts it succinctly: "There seems something vaguely unsavoury about talking about my son with Down syndrome when promoting a show". I was however curious about why she shouldn't want to speak about her religious convictions.

"I suppose you ask yourself why we are sitting here and what it's for," she says after a long silence. "My personal experience is that about 60 per cent… the majority have some form of violent reaction to it. So is it wise? Who does it benefit? You make your own assumptions if you read 'Sally Phillips is a Christian'. You'll think 'Oh she's definitely homophobic or anti-woman bishops'… I mean, are we going to have an hour-long discussion about exactly where I stand on certain doctrinal issues? No, because we're promoting a sitcom."

Well, that's sort of what we're doing – although Miranda hardly needs promoting. My side of the bargain is surely to give readers an insight into what makes Sally Phillips tick. "It's not that I'm ashamed of it," she says. "I tell people that I'm a Christian, but I don't think it's giving an insight into who I am or what I'm about. If I'd written a film – and one day I will – where Christianity is an essential part of it, then it would be a much more appropriate thing to discuss."

In fact, Phillips turns out to be much more straight-talking than the late-imposed preconditions to our interview had suggested, and it would be a shame if she ever acquired what she calls a "mouth-gate". With a first from Oxford, she's obviously highly intelligent, her answers always thoughtful, as when I express to having been surprised when Miranda's upper-middle-class milieu first found a mass audience. "I would say exactly the opposite," she says. "We've got Etonians running the country… for some reason that's acceptable to people… we've got Downton Abbey, massive, and it occurred to me that it's quite similar to the screwball era when women were allowed to be funny because they were all heiresses. Obviously Miranda is very funny, but maybe 10 years ago she wouldn't have got her own series. For some reason being posh is all right."

Phillips herself claims to be more déclassé than posh, thanks to an itinerant childhood following her father and his job as an executive with British Airways. Can she rattle off the countries she and her two brothers lived in before settling back in England at the age of 13? "Hong Kong, Zambia, Brunei, Beirut, Abu Dhabi, Dubai, Bahrain, Australia, Italy," she rattles. "It wasn't a type of Raj upbringing. My mum's from Yorkshire and my parents aren't snotty or posh – they're very hard workers, both of them."

Her father, Tim, became the chairman of the All England Lawn Tennis and Croquet Club (home of Wimbledon tennis) when he retired from BA; her mother, Elizabeth, a tough-sounding, well-regarded head teacher – currently at St Marylebone Girls' School in London (a photograph of her on the school website shows a strong resemblance to her daughter) – had her first job at Feltham Community College. "It felt like a feeder for the Feltham Young Offenders' Institute," says Phillips. "She came back beaten up and I remember just crying and saying, 'Stop doing that'."

Phillips herself was sent to a rather more genteel boarding school in Gloucestershire, where she failed to fit in. "I wasn't a member of the pony club, and I didn't have any clothing from Benetton or any shoes from Hobbs, so that made me unusual. The question is whether it's good or bad to be unusual in that environment, and now I think it was great."

She had a rather more congenial time at New College, Oxford, where she got a first in Italian and started writing and performing comedy for the Oxford Revue, with the likes of Al Murray, Stewart Lee and Andy Riley, the creator of the Bunny Suicides series. "It attracted the geeks… the real comedy spods."

After university, Phillips paid her dues at countless Edinburgh Festivals. "It was a macho world," she recalls. "I definitely internalised the idea that women aren't funny. I remember one year leafing through the programme and going, 'There's loads of girls going up on their own… maybe I should do that' and the guy that If was writing with/for at the time said, 'What would you call it, The Sally Phillips Experience? And he laughed for about 10 minutes, and I joined in laughing and then went into another room and cried."

In 1995, she teamed up with Simon Pegg and Jessica Hynes on the regional ITV sketch show Six Pairs of Pants, reuniting with Pegg four years later on the short-lived sitcom Hippies, set in an Oz-like 1960s magazine and written by Father Ted creators Graham Linehan and Arthur Mathews. But by now she had broken through – first playing the giggly receptionist in I'm Alan Partridge and then with Fiona Allen and Doon Mackichan in the ground-breaking, all-female Channel 4 sketch show Smack the Pony. "I work now with women 10 or 15 years younger who all grew up watching it," she says. "As a teenage girl it wouldn't occur to you now that women aren't funny. It's a definite career choice and there are loads more girls. There really weren't any then."

Phillips is hoping to reunite with Allen and Mackichan in a new sitcom called Stick or Twist, about three sisters whose marriages are in crisis, although she's still waiting, after nearly a year, for the BBC to give it the green light. "It feels like the system has shut down a bit," she says. "TV feels quite constipated and the thing I find particularly difficult is the branding of the channels where it's not 'Is it a good script?' but 'Is it a BBC2 script?'."

Stick or Twist sounds like a timely idea, for as Phillips says, a propos of Smack the Pony: "It did change things but I think there are still some things that can be changed. Middle-aged women on telly is a bit of a hot topic – before we were 27 to 37 and now we're 40 to 50. You do notice as you get older… you go past 35 and suddenly you're playing baddies. I get offered people who get killed in slasher movies. Where's that come from – haven't they read about me being a Christian?"

It makes a change from being forever cast as 'the best friend', a process that continues with Miranda, but which began with the two Bridget Jones movies, in which she played the wine-guzzling singleton's pal, Shazzer. "Never the lead, always the best friend," she says of the offers that came in post-Bridget Jones, adding that she can't enlighten me as to the progress of a long-promised new sequel. "All I know is that I was booked, the dates were pushed and pushed and then they called it off but they said we still intend to make it. That's all I know.

"We got on very well," she says of her co-star Renée Zellweger. "We were in touch for a bit but we were quite honest about it – she's a massive movie star in LA and I was pregnant with Ollie on the second film. When I had Ollie we talked a lot and she was incredibly kind when he was born, but you know, you're just aware when that happens that your life is going to take a slightly different path."

Phillips isn't interested in joining Zellweger in America – or rather isn't interested enough, even though she retains a Hollywood agent. "What attracts me about over there is that TV drama and TV comedy is the best in the world, but it seems like a very brutalising process. Every spring there are these planes full of British actors – it's like sardine-tins full of British hope – and they all go and sit on beaches and think it's fantastic for a couple of weeks and then get fantastically depressed and go on juice diets and come back two stone lighter and talking nonsense.

"The last time I saw Eddie Izzard he said, 'You've got to come, you've got to come, but you've got to want it more than blood' and I really had no idea what he was talking about. I thought, I don't think I do want it more than blood – what does that mean? Does it mean I want it more than my family?"

Her family includes her three sons and husband Andrew, who works in shipping, and not, as the cuttings state, in IT. "The things they've printed have been hilarious," says Phillips. "He's also been a Bible teacher, which I enjoyed. His family have enjoyed taunting him about the Bible teacher bit and his colleagues have enjoyed taunting him about the IT bit. He's a total techno fool like me. We can neither get anything to work."

The cuts also say that Andrew is Spanish – unusual Iberian nomenclature, I suggest. "He's eurotrash… No, I shouldn't say that – he's a mixture. He's Spanish, Italian, Scottish, German, Jewish mix. His first language is Spanish but he came back here when he was 13… that's the glaring similarity between us."

They live in Richmond, an easy commute for BBC Television Centre – where those teenage girls are queuing around the block to watch Miranda being recorded – and Broadcasting House in central London, where her Radio 4 comedy Clare in the Community, in which Phillips plays a jargon-spewing social worker, is made. But it was Bush House on the Strand, until this summer the home of the BBC World Service, which inspired the most reverence in Phillips when she visited. "Growing up, we had the World Service on 24 hours a day, so the first time I walked into Bush House to do a radio show I was really over-awed. Home of the World Service… 'Broadcasting from Bush House', all of that…". How could Hollywood ever compete?

'Miranda' continues on Mondays on BBC1, and 'Clare in the Community' continues on Wednesdays on Radio 4