Nick Frost says British TV likes farce and it's great to play a "nice, dark idiot".

What appealed to you about Sick Note?

I like dark comedy, and this is definitely dark. I like the fact that it's brave in terms of its subject matter. But above all, it's been great fun. Working on this has been an absolute hoot.

How would you explain the premise of the show?

It's about a lie that spirals out of control. If there is a moral, it's just be honest. But I like the fact Daniel takes it so far just for an easy life. There are people like that in real life.

Tell us about your character...

Dr Iain Glennis is probably the worst oncologist in the world! I love the fact his life is so awful. He is lost and broken on every level. Sadly, that comes out in his work when he misdiagnoses Daniel.

Just how appalling a doctor is Iain?

He is so bad at his job that he can't even pronounce the word oesophagus. We did a scene the other day where he tells Daniel, "You have cancer of the asnophagus". I like that farcical aspect of the show. That's a good thing for British TV. British TV likes farce. This is a great chance to play a nice, dark idiot.

I mean, how on earth has this guy not been struck off? It's actually quite scary. There are a few places where he alludes to the fact that he may or may not have killed lots of people - "Listen, they were all patients - at least most of them were patients!"

How do you think he became a doctor?

He's from a moneyed background. I am playing him a bit posher than I am. I don't think he could get into medical school in Britain, so his family paid for him to study somewhere else. Of course, he would only go to classes once a week. He would do nothing all day - except go to the driving range because he's posh! But because he's from a privileged background, he's managed to get away with it.

How would you characterise Iain's relationship with Daniel?

They're locked together. I have a feeling that Iain doesn't have many friends. So oddly, he sees his relationship with Daniel as a friendship - "All right, we're in this together now". He is constantly patting Daniel. They've been through a lot together.

But it's a horrible relationship. They've basically been forced together, like Robert De Niro and Charles Grodin's characters in that great film Midnight Run. Iain and Daniel have Midnight Run Syndrome, where they have been obliged to become close thanks to this terrible mistake.

How have you found working with Rupert?

He was the only one of the big Harry Potter three that I hadn't met. He's lovely, funny and clever. It's very nice to come to the studio and know you'll be working with someone who knows what they're doing. Rupert does giggle a lot, and it's been fun trying to make him a laugh during takes. It's been a really good partnership.

What else is going on with Iain?

He is in a dreadful relationship with his wife Annette [played by Belinda Stewart-Wilson], who is a horrible bully. But I have to say, being shouted at by Belinda has to be one of the highlights of my career! Annette is constantly badgering him. He is terribly henpecked. When she is being so horrible to him, he just stands there and takes it. There are no redeeming features in their relationship!

You get the impression that she's hit him a few times. There's been blood. I reckon every New Year's Eve ends up in carnage. We also flirt with the fact that he's potentially been cuckolded, and that just makes him all the more sad!

Are you concerned people might get the wrong end of the stick about Sick Note?

No. We're not taking the mickey out of people with cancer. We're taking the mickey out of people who lie. Daniel has jumped on the fact that he could use this diagnosis to his advantage. That's where the comedy comes from.

Finally, are there any similarities between you and Iain?

None whatsoever! Iain is the least me of anyone I've ever played. I've occasionally slept in my car like he does - but for different reasons! A couple of years ago, I went through a phase where I'd go for a drive in the evening and then find myself pulling over to have a nap by the side of the road. We have that in common, but that's where the similarities end!