Sally Phillips: from Alan Partridge to The Decoy Bride

We're in the Malmaison and Phillips is with her baby son. She tells me that her other two boys are back at home in Richmond in London, being looked after by a nanny because that's what middle-ranking comedy actors can afford, and talking about the previous night's UK premiere of her new film, The Decoy Bride, in which she appears and co-wrote, at the Glasgow Film Festival.

The word she uses is "relief" to describe the fact that the audience seemed to enjoy the film. Later she will tell me that she's no great beauty (or rather she will tell me that someone once told her she was no great beauty, but it possibly amounts to the same thing), and that, "relatively speaking", she did an easy degree at Oxford (Italian). In short, she's not one to blow her own zampogna (Italian bagpipes. And yes, of course, I've just Googled that).

These days Phillips is probably best known as the posh best-friend, supporting-character type. She was the posh best-friend type in the Bridget Jones films and she's the posh best-friend type in BBC One's hugely successful sitcom Miranda. She's good at it, although I always preferred her as the smirking Norwich Travelodge receptionist who struggled (and mostly failed) to hide her contempt for one of the guests, DJ Alan Partridge. Or as one of the Smack The Pony team, the all-female comedy sketch trio which generated their own material and were one of the pleasures of Channel 4 at the turn of the century. In short, given the chance, I think she should be more than just a posh-friend character type. She is much funnier than those parts deserve.

If anything, you could say she's been a bit unlucky careerwise. Her part in Notting Hill ended up on the cutting room floor. A role in Terry Gilliam's Don Quixote disappeared when the film was cancelled before it really began. She once got a lead role in a comedy drama, Rescue Me, written by none other than David Nicholls. But this was before David Nicholls was One Day's David Nicholls, the show was watched by hardly anyone (3.4 million people being hardly anyone on BBC One in Sunday night telly terms), and she disappeared back into supporting casts.

She's also had the disadvantage of living in conservative times. TV – or British TV at any rate – hasn't really been that keen on taking risks in the last 10 years. "I had a desperate meeting with the BBC five or so years ago when they said 'we really want you to write something for us. But it has to be for BBC One, it has to be a studio audience sitcom and it has to star Martin Clunes, Dawn French, Sarah Parish or Jimmy Nesbitt'. And I could almost feel the hands around my throat. I felt strangely uninspired because there's no room for you to say 'oh, I'm really interested in drugs', or whatever it is. There's no chance for the thing to grow."

The Decoy Bride – a Scottish screwball comedy in which David Tennant marries Kelly Macdonald by mistake – is itself a source of some frustration. It should have cost £7 million but, as she points out, "a very, very clever director [Sheree Folkson] has managed to tell the story for £2.5m". More money or time wouldn't have hurt, she admits. But at least it got made.

When we met on the set of the movie 18 months previously, she'd told me that if you wanted to dig a bit deeper the movie was an argument about "the homogenisation of desire".

"If you get 10,000 guys to put their ideal woman into a computer, it still comes out looking like Angelina Jolie. Whereas, if you ask women, it comes out as this strange alien creature because someone's put in Woody Allen, David Tennant and Arnold Schwarzenegger. It just doesn't even look human. This is a problem for girls."

Don't fret by the way. People fall into moats in the movie too. Actually, Phillips, as a writer and a practitioner of comedy, is a good person to talk to about the very nature of the artform. I tell her that I like my comedy to be about wordplay. A colleague insists that comedy is simply a matter of falling over in a funny way. Who's right? "It's absolutely possible to love both," she says. Spoilsport. "There's an exhilaration with slapstick," she continues. "I would love to have been around in the Keystone Studios days."

And then she gives me as good a defence of comedy as you're going to get: "Everyone is ridiculous, basically. It's just a question of finding out what is ridiculous about each person. It's normally the thing they hold most dear. Some people are not prepared to expose that to ridicule. But the more serious someone is, the funnier it is watching them walk into a wall, I think."

Sally Phillips was born in Hong Kong. Not that she can remember it. Obviously. It was 1970. There are some childhood photographs of her and her brother sitting on the balcony overlooking Hong Kong harbour with the RMS Queen Elizabeth burning merrily in the background. Her father, Tim Phillips was an executive with British Airways, and later chairman of the All England Lawn Tennis and Croquet Club, Wimbledon, so she probably doesn't have to look too far for material for those posh best-friend parts, I'd imagine.

Her childhood was spent constantly on the move, to Italy, Australia and the Middle East. As a child she was in Beirut at the same time as Dom Joly, star of Trigger Happy TV (which was airing about the same time as Smack The Pony. "It's really bizarre that two contemporary shows on Channel 4 came out of the Beirut primary school system," Phillips says). "He ended up abusing me for being a coward and leaving when Syria started dropping bombs. He stayed. Apparently his whole family stayed in a cellar throughout the civil war."

Eventually, in an attempt to give their children stability, her parents sent them to boarding school. Wycombe Abbey. She didn't care for it. "No. Didn't like school much. Not really, no." Why not? "I didn't like being away from home actually. Sounds a bit wet. We're now all really close. We all live within five minutes of each other. It seems strange now that we went to school in a different country [her parents were in Australia while she was in England]. It was quite hard to pick up the culture, the whole 'really being interested in Smash Hits when you have no access to Top Of The Pops in the holidays'. I had all the wrong clothes. My mother converted all her 1970s' clothes. When we were in Italy it didn't matter that I was wearing 1970s Jaeger clothes turned up. But in England that really, really mattered."

She likes to recast this time as a life lesson. "I became more comfortable with the idea of not being the same as everybody else, which is something everyone really needs." Perhaps, but she concedes that, as a teenage girl, "it was a bit rough". Was she picked on? "Yes. A little bit, yeah. Nobody hit me. They were a bit pathetic. They used to do things like spit on my homework."

Still, school gave her an education good enough to get into Oxford University. And she loved it. Absolutely loved it. "You're probably not allowed to say that. But it was such a privilege and a blessing to be exposed to that kind of very extreme thing.

"Everything's really intense. The work's really intense. My cousin did the same course as me at Bristol and she did two essays a term while I was doing two a week. And everybody does something else. I did acting."

She started performing at the Edinburgh Festival when she was 18 and did so for nine years (with one off in the middle). She never did the big rooms, she points out. "I was always in smaller rooms and they were usually full and the air was thick. Everyone's glasses were steaming up and you're knackered because you've been up until four in the morning at Late and Live watching comics fight each other and women be sick on their shoes.

"Apparently it's completely different now. It's all much more professional. It wasn't terribly professional when I did it." It proved to be a good grounding. But in her late teens she had another dream. She quite fancied a career in Italy. As part of her degree she had lived in Rome. She even got herself an Italian 'equity' card. "I got an agent out there and he said to me 'Sally, Sally. You are no great beauty. Go back to England and come back to me when you're Emma Thompson'. That was 1992."

There's that self-deprecation again. It reminds me of a comment she made when I met her on the set of The Decoy Bride: "Comedians have to write to survive because you don't get cast for your beauty." At which point it might be worth saying that, despite what Phillips believes, not everyone with the XY chromosome fancies Angelina Jolie.

Anyway, when she reached a mild level of fame in the wake of Smack The Pony about 2000, she toyed with the idea of returning to work in Italy.

"Italian cinema has highs and lows, but it's still a force, isn't it?" She still rather fancies the idea but now she says her Italian is terrible: "I married a Spaniard so it's completely corrupted."

Instead she lives in Richmond, with said husband, IT specialist Andrew Bermejo, and their three sons. She walks in the park, pretending to be Lois Lane, while her four-year-old son is Superman, and goes to church. But she doesn't want to talk about that.

"People have really strong images of what church is and it's almost certainly not the same as mine." Well, it's yours that I'm interested in, Sally, I point out. "I don't know. It just gets misinterpreted. It's a very important part of my life, my faith, that is usually misunderstood even by people within the church. It's something I'd rather shut up and just do. Somebody said 'preach the gospel at all times and if necessary use words'. And I feel like when I use words it comes out wrong and it is misunderstood. I would really rather not talk about it and just make my blundering way through life getting it wrong."

She's proud of The Decoy Bride. Time will tell if it will open new doors for her. She's hopeful. She thinks TV is beginning to open up to new ideas. "When I started doing this, it felt like television had gone a bit dead. It felt like it was being squashed from above. Executive producers had proliferated and there were too many people involved in every decision. And they were zombies to the American way of doing things. It feels like that's cut back now."

The arrival of Sky as a serious player has helped. It gave Gavin And Stacey's Ruth Jones 10 hours to play with, and she came up with Stella without having to show them a single script. That's encouraging. "There needs to be a freedom to fail. That's the thing. In a culture where there are too many executives, there is no experimentation."

However, whether TV or film will ever be truly ready for Sally Phillips I'm not sure. What's it like inside your head, Sally, I ask. "It's a cross between Boethius, Gormenghast and Arcimboldo. Basically very metaphorical. I live in terms of metaphor."

She tells me a story. Ten years ago the then BBC Two director Jane Root commissioned a pilot from her. "Make anything you want," she said. So Phillips came up with a lower middle-class Gormenghast about a monster in the sewers in love with Mickey Blue Eyes who in turn is six-timing sextuplets who, it turns out, are actually the same girl who keeps inventing siblings every time she gets dumped. It was, she admits, quite surreal and rather odd. Root, unfortunately, wasn't looking for surreal and odd. "She said 'but I thought you were going to do a single girl looking for love in a cappuccino bar'. She was so sure that's what I'd come up with."

These days Sally Phillips plays posh best-friend characters. I surely can't be the only person who'd like to see her given the chance to tap into her love of early Christian philosophers, English neo-gothic fantasy and Italian painters of portraits made up of various bits of fruit and veg. And if you ask her nicely I'm sure she'll find a part for Jimmy Nesbitt.

The Decoy Bride (12A) is released on DVD on March 12.