Interview: Posy Simmonds, cartoonist

• Tamara Drew as she first appeared in The Guardian

It began life as a serial in the Guardian and has been transformed into a film by director Stephen Frears, working from a screenplay by Moira Buffini, writer in residence at the National Theatre Studio.

This tale of love and lust, art and artifice, truth and deception, set in a claustrophobic rural village that's home to a writer's retreat, has been cast with an uncanny eye for verisimilitude. Though Simmonds couldn't have had the then unknown actress in mind when she created Tamara, Gemma Arterton is the spitting image of her eponymous heroine.

Likewise, Luke Evans is every - glorious - inch the hunky handyman, Andy Cobb, and bearded Bill Camp is spookily accurate as writer Glen McCreavy. Roger Allam is suitably smarmy as womanising detective novelist, Nicholas Hardiment, while Tamsin Greig as his long-suffering wife, Beth, who does all the real donkey work keeping their retreat going, breaks your heart a thousand different ways.

I mention this to Posy when we meet at her London home a few weeks after her return from Cannes, where the film premiered. "The casting is amazing," she agrees. "Tamsin Greig is simply wonderful in a very difficult role. I think she wore a fat suit, and she was given the most terrible wig, and this awful wardrobe of terrible old sweaters and things like that."

That is Simmonds' fault, of course, and a testament to her powers of observation. "They're all wearing what I drew them wearing, more or less. Before I do anything, I draw characters and create a wardrobe. I do spend quite a lot of time dressing them, so that was nice." If you know the book, watching the film is eerie, for not only have clothes and interiors been accurately recreated, some of the shots are identical to her drawings.

Knowing Simmonds, 65, to be a marvellous raconteur with exceptional powers of mimicry, and someone all too eager to prick pomposity, I'm dying to hear how she fared at the film festival that is synonymous with gaudy excess.

"It was hysterical! Because of the volcano and various threatened strikes, it meant trying to meet in Belgium for starters. I got up at 3am and a car came at 3:45am. We were absolutely whooshed through Heathrow. When we arrived in Nice, around lunchtime, they sent limos to meet us. I imagined having at least half an hour to put on the slap and ponce about in the hotel. In fact I had about four and a half minutes in somebody else's room because mine wasn't ready."

With noises rather than words, Simmonds conjures the splish splosh of a swift sponge bath. "This was a feng shui hotel, which meant there was trickly water everywhere, which made everyone want to wee. Also there was this water effect in the lift," cue glugging noises, "and everything was in twilight. Luckily I had a torch in my bag. And then eventually when I got a room - the whole hotel was divided into continents, and I was in Africa. You couldn't see a thing so it really was the dark continent. I had to use my torch to see the numbers on the rooms.

"Inside, it was all hung about with saris, so if you wanted to read in bed, you actually couldn't see with these bloody things everywhere. I spent a lot of time drawing the hotel and what the staff wore - it was so wanky, and overdesigned."

Stepping out of the hotel's gloom into bright sunlight "was like being slapped" . They were transported, by "limo-cade", to the screening. "The limos have got flags in the front! Oh God, it's both cringeworthy and also exciting."

It was the first time she'd seen the film with a proper audience. "They laughed like drains," says Simmonds, genuinely delighted. "It was wonderful. Moira has some very good jokes. The film is less sombre than the book, especially at the end, which is slightly changed."

The crowd loved the film enough to give it a standing ovation. Next, Simmonds and company were whisked to an enormous villa way up in the hills, "where there was the most splendid party. Champagne, and people with terribly good clothes and shoes. You had the wonderful sense that so much business is being done at Cannes. There were a lot of quite fleshy men of about 55, but breeding condition, and tanned, with very white shirts. They were wearing very, very good suits - no ties. When they meet women - who all look like racehorses - it's a very polite greeting. But when they meet one of their own kind, it's like a couple of bears - they cuff each other on the arms." She clasps and unclasps her own arms roughly, just in case I don't understand.

Like the audience in Cannes, I laughed endlessly at a screening I attended, marvelling, as well, at the strong, sharp performances. Dominic Cooper, playing Tamara's make-up wearing, rock star boyfriend, is wonderfully boorish and not much more grown up than the talented newcomers - Jessica Barden and Charlotte Christie - playing the meddling teens whose misbehaving sparks off so much of the action in the film. But one critic who saw the movie in France complained that it was too jolly.

Simmonds says: "I think people need a good laugh these days. When you think of British films you either think of aristocrats prancing about in jabots in National Trust properties, or at the other end, it's gritty, life is really a s*** sandwich stuff. We don't see many films about the middle class. And life is sad at the moment, so a good laugh is nice. But I can see that perhaps, in England, although people aspire to the bourgeoisie lifestyle, with gardens and Agas and all these things, it's also slightly to be sneered at."

This opportunity to document the British bourgeoisie appealed enormously to Frears. "I was conscious that Chabrol, and the French, make films about the middle classes, and that French directors like Pagnol and Renoir made films about the countryside. I like all of that, for silly reasons, because nobody else does it. So you're immediately doing what nobody else does. That also enables you to get out of bed, and it just doesn't bore the backside off everybody."

Frears and Simmonds first met around 40 years ago, though she wasn't instrumental in his hiring. "We had a mutual friend, who is dead now, and met through him. Because this friend was chronically ill and often in hospital, he had like a salon at the Middlesex. He'd ring up before you'd go in with very specific orders: 'Could you bring me some bagels and smoked salmon and coffee, please?' A very refined man. He knew everybody. And I used to meet Stephen at parties and we'd have a laugh. I've always been a fan of his movies."

She visited the shoot on location in Dorset for two days, but mainly kept her distance, and insists that this is very much Buffini's project. "I realised what it was like being a script writer, and it's not something I could do. Moira is extraordinary. Stephen would say, 'I think that scene needs to be shorter', or something like that - which is quite complicated, you know, the weight of it, the meaning," Simmonds makes a gurgling noise which I take to signify the revision process, "and she would go away and tinker and back it would come fixed."

For Buffini, having the graphic novel to refer to was akin to having a ready-made storyboard. "Visually you've got so much there. She gives you so many clues to the character in her drawings." Frears found this useful as well. "Frequently we would do things and you'd look at it in the book and say - 'Well, I can't improve on that. It tells you everything you would want to know.' Somebody before you has compressed everything down to a single image."

When Simmonds selected her Desert Island Discs in Radio 4 in 2008, she told Kirsty Young that she started drawing as soon as she could pick up a pencil and as a child was making magazines and little comics with titles like How to Turn Yourself Into an Up-to-Date Ted and How to Make Love and Be Loved in Four Easy Lessons. She said she remembers drawing as the perfect thing to do, because she could sit on her own and talk to herself.

It's been a lifelong passion. She's already working on her next project, though the subject matter remains top secret. It's early days, yet, she tells me. "It's always very slow at the beginning. I've learned the phrase now - it's in preproduction. I want to get the details right - if you do that, it makes everything else convincing. For instance, is it someone who always stands with their toes in, or they have a hot water bottle even in the summer?"

One last detail, then about Simmonds: she's had her computer less than a year, and isn't a huge fan of e-books. "You can't go to sleep under a Kindle. I love a book on my face - after you've had a good read, and you're slightly dozy, the lovely smell of paper ... mmm. And what you do to a book, too. Say it's a long one and you've travelled with it, and later you open it and sand falls out, or you find a seagull feather and a lolly stick and chocolately thumb prints ... I'm terrible!"

Tamara Drewe (15) is in cinemas on Friday.

Acute observation and wry humour is at the heart of the work by cartoonist Posy Simmonds, whose witty take on Far From the Madding Crowd has now been given the big screen treatment by Stephen Frears

This article was first published in The Scotsman on Saturday, 4 September, 2010