There's no one who does put-upon quite as well, or comically, as Katherine Parkinson. It's that pale face, those big, swimmy eyes and, most of all, that nasally whine – "Not," she points out, "a million miles from what I was born and raised with" – that somehow make you laugh at her even when you feel you really shouldn't.
More often than not, her comic characters come with a whiff of the tragic, or at least the downtrodden. In The Seagull at the Royal Court, she skulked around, a maudlin, alcoholic Masha to Kristin Scott Thomas's glamorous, wafting Arkadina, while in Season's Greetings last Christmas she stole the show from Catherine Tate as Pattie. She's best known for playing Jen, the unnecessarily power-suited manager of The IT Crowd who spends her days in a dingy basement, dealing with deadbeat geeks in a job she neither understands nor wants. "I suppose I'm less interested in comedy for light entertainment purposes," she says. "When things make me laugh it's not in a cosy way. It's in a slightly horrific way."
Her taste for the slightly horrific has seen her work with just about every major name in British comedy – Graham Linehan, Jesse Armstrong and Sam Bain, Ricky Gervais, Richard Curtis – over the last decade, picking up a British Comedy Award (and a Bafta nomination) for The IT Crowd on the way. Now 33, she has no interest in what she calls, quietly dismissive, "Mogadon telly", playing a ditzy housewife in a gentle, Sunday night sedative-sitcom. Her dream collaborators are Mike Leigh or the macabre playwright Dennis Kelly. "It's not that I don't want to do the things that are popular across the board, but there's a type of tame comedy that I think is a cop-out. I prefer a less polite laugh."
Today, huddled on a blustery park bench in Blackheath, the green south London suburb where she lives with her husband, the actor Harry Peacock, she is the picture of politeness. Delicate, with porcelain features and only a hint of nasal twang, she takes up a lot less room than her larger-than-life characters. Like a lot of comic actresses, she's rather shy when it comes to appearing as herself. She recently had to present a Bafta and "got in a right tizz" because she didn't know how to pronounce "behemoth". "When it came to it, the weirdest sound came out... Behammmmeoth," she honks with laughter. She was similarly het up when her good friend, the comedian Katy Brand, asked her to read a poem at her wedding. "That's when I get funny because I feel I could be judged to be a prat. It's why I don't do panel shows. I will almost certainly get so embarrassed, I'll make an idiot of myself."
Otherwise, she's a consummate professional when it comes to making an idiot of herself. She's been doing it for years, ever since she was cast as Puck in the school play, aged 13. At drama school, she was the class clown, forever cast as Feste and fooling around with her best friend and future IT Crowd co-star, Chris O'Dowd. "We got on because he was quite irreverent and naughty and so was I. When we had to do stretching, he'd pretend to have pulled something..." Looking silly comes naturally, then. but these days she takes it rather more seriously. Her Lady Teazle – with towering wig and heels - in Deborah Warner's revival of The School for Scandal earlier this year was a masterclass in clowning. "I'm not someone who goes home and looks in the mirror to work out which face to pull," she says. "But what I don't like is being given shoes at the last minute, which completely transform the way you walk and talk. I had a drama teacher who would say, 'You must start with the shoes' and I remember thinking, 'that's silly'. But you absolutely should. I like to know how I'm going to feel physically."
Does she ever get embarrassed? "Oh God, yes!" Performing a sex scene (albeit one enacted in dialogue) with Ben Whishaw in Cock at the Royal Court two years ago was a challenge – especially on the night her parents came to watch. She'd told them that the play was called "Cock-a-doodle". "The trouble was, it was in the round and everyone was lit so you would catch somebody's eye mid-climax or whatever," she trails off. "It was a real exercise in concentration."
Parkinson grew up in Tolworth, Kingston upon Thames, and went to the Tiffin Girls' School before reading classics at St Hilda's, Oxford, then an all-women college. "It was a bit of a soft option – a weird mix of these brilliant, proto-feminist women tutors and slightly flaky people, like me." Having only appeared in one play at school, she embraced the drama scene with gusto – "I was gagging for it, so to speak." She did endless plays, including a version of Iphigenia at Aulis in Ancient Greek. "I had a big green smock and some slippers. I looked like Friar Tuck. I said the same line quite a few times, went into pigeon Italian to buy time... Awful."
After graduating, she went to Lamda where she finally learned to take comedy acting seriously. "I spent my time thinking it was silly but by the end I could talk about my motivation without pulling a face. I'm pleased it was beaten out of me." In her first summer, she performed in The Age of Consent at the Edinburgh Fringe, which transferred to the Bush. A term later, Josie Rourke came in to direct a student She Stoops to Conquer and plucked Parkinson out of lessons to appear in Frame 312 at the Donmar. Eventually, Lamda gently suggested that she might as well leave and start her career. She's worked ever since.
She was "terrified" when she was cast in The IT Crowd five years ago. The sitcom, written and directed by Graham Linehan (Father Ted) and produced by The Office's Ash Atalla is now four series in, with a devoted following who shout lines at Parkinson in the street, to her bemusement. Filmed in front of a live audience, it's less acting than extended improv comedy. "Lines change right up to the last minute. You have to have balls of steel, and own it." A Christmas special and a film are planned. "I've heard Graham's ideas and they're really clever. It's not the sort of show where you can do an Inbetweeners and have them all go off on holiday." A US pilot has also been filmed, with the American comedian Jessica St Clair as Jen. "She looked much better than me, which, frankly, I would expect in LA. I would have been insulted if they'd downgraded me physically."
Her two main co-stars have both moved into film this year – O'Dowd as the love interest in Bridesmaids and Richard Ayoade as the director of Submarine. Parkinson is keen to follow them – "I worry about people getting fed up with my face on television, being too associated with a part." She's thinking about going to LA next year, if she can bear to leave her husband, who is currently starring in Linehan's version of The Ladykillers in the West End. They met in 2003, workshopping Heart of a Dog at the National Theatre studio: she played the cook, he the dog. They married three years ago. "It's difficult when you're both actors. I almost wish I was with an actor who didn't work but he's got a good career, too. We're not a low-maintenance couple that can see each other once every four months. I need to tell him everything that has happened to me. He's the best thing that's happened to me and I'd quite like to enjoy it. At the same time, I am as ambitious as the next actress and I don't want to cut off my nose to spite my face. If they didn't write such damned good stuff, it would be fine but the comedy over there is so accessible."
Before that, she's returning to Ayckbourn and the West End next month to play Diana in Absent Friends, a typically acerbic take on adult friendships, starring Reece Shearsmith and Kara Tointon. Over Christmas, she appears in The Bleak Old Shop of Stuff, an extremely silly four-part Dickens spoof for the BBC, as Conceptiva, the randy, ringlet-ed wife of Robert Webb's Jedrington Secret-Past. David Mitchell, Stephen Fry and Johnny Vegas also star. It's another male-led comedy but times are changing. The global success of Bridesmaids means there is a buzz to female-led comedies and Parkinson is top of the lists to star. "Needless to say, people want to do that here now," she says, raising an eyebrow. "You want to say, 'Great, but please let's not just do 'Hen Night'."
A cerebral soul, it's unlikely that she will sign up to anything she is unsure about. She's barely put a foot wrong so far . So does she have any regrets? "Although I didn't think twice at the time and it's great, I'm a bit of a snob..." she says, at long last dropping into her signature comedy whine. "So when people say, 'Weren't you in a Maltesers advert?' I just want to say, 'And Chekhov!'"
'The Bleak Old Shop of Stuff' starts on 19 December at 8.30pm on BBC2; 'Absent Friends' is on at the Harold Pinter Theatre, London SW1 (08448717622; www.absentfriendstheplay.com) 26 January to 14 April