RUPERT GRINT is recalling the worst lie he's ever told. "It's recently been discovered..." he begins, "But there was a Mr Blobby music video in the 90s and there's a kid in it that looks exactly like me and I used to tell people that that was me. It wasn't me at all!" he quips. "It was three years before I was probably even born and someone did the maths, but I convinced myself it was true!"

But the Harry Potter actor, 29, isn't here for a confessional. I'm simply gauging his morality in comparison to his chaotic character in Sky One's brand new comedy-drama, Sick Note.

Grint plays compulsive liar Daniel Glass who, stuck in a failing relationship and a dead-end job, is lacking a lust for life. Until, wait for it, he's diagnosed with oesophageal cancer - or misdiagnosed as it soon becomes apparent - by the less than competent Dr Glennis (Nick Frost). Taken aback by the sudden care he receives from those around him, Glass's decision to continue to fake his illness, rather than come clean, soon sees his life spiralling out of control.

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"It's an interesting one. It's a difficult subject to cover and make a comedy about," says Grint, who has already filmed a second series. "It's quite dark and I was worried when I was first involved with it: I thought, 'How are people going to react?' But I'd never read anything like it.

"I mean, it's sensitively done. It's not making fun of cancer or making light of cancer. It's portrayed as a terrible thing and it's not undermining that; it's a comedy about lying and how far someone will go. It takes a whole new direction and that becomes secondary in the end.

"Even filming it was quite stressful," he says, adding: "It was quite hard to track where the lies were going and what I was lying about. It did my head in a little bit."

Of the scene when his character is diagnosed, he confesses: "I thought, 'S**t, this is exactly what it would feel like'. I can't even imagine... Actually before we started filming, I thought I had - I'm a bit of a hypochondriac anyway - melanoma!

"It was probably because Sick Note was in my head," he says. "I just thought, 'This would be so classic if I get cancer when I'm doing it'. It would be so weird. That's why it's such a taboo thing - you don't want to face it, it's such a scary word."

For Essex-born Grint, whose comedy roots include Urban Myths and Tracey Ullman's Show, the titular role is a departure from his early days on the big screen. Cast in the Harry Potter film series at the age of 11, the former child star is best known for featuring in all eight of the franchise's films as the fiercely loyal Ron Weasley. It's a part that, while life-changing, has meant he's had to battle harder to avoid being typecast.

"That's always going to be the challenge," he muses. "It's the nature of those films: they were watched by so many people, the books were so loved and these characters, people are really invested in them and they feel ownership to them. So yeah it's always going to be a fight. But it's never something I'm really acting against. It will always be with me. I am proud of it and I was really proud to be a part of it - it was a huge part of my life. It's never died down as much as I thought it would, but I don't mind it too much. Sometimes you just want to be invisible, but it's just part of it."

He refers to watching himself grow up on screen - between 2001-2011 - as a "really expensive home video".

"We still all went through the teenage phase and there were times when I wanted to quit and rebel, but it was never really serious," he confides. "It was a weird way of growing up, to be so much in the public eye as well, and having that thrust upon you so suddenly was a real adjustment."

Of how it's affected him, he says: "It kind of makes you a little bit more reserved and you're a bit more cautious of people as well, which are some of the inevitable side effects. You just want a bit of privacy. Maybe that's why I stay away from social media.

Is he still in touch with his Hogwarts co-stars, Daniel Radcliffe and Emma Watson?

"Yeah, occasionally," he responds, with a smile. "We spent so much time with each other and we grew up with each other, so we'll always have that special bond. We don't really need to see each other all the time, but we're still in touch. It's fun to see what they're up to."

For now Grint is simply trying to come to terms with turning 30 next August - "It just sounds like a mad number, doesn't it! I still feel really young, I don't feel my age," he declares. "It's a weird contradiction, because on Harry Potter you were in a real adult world where you're spending most of your time with adults, but at the same time I've never really felt adult. I always feel like I've been kept behind a few years because everyone does everything for you. The responsibility, an element of that is kind of removed."

But with his sights set on more TV and a starring role in the first and second series of Snatch - a drama loosely based on the popular movie of the same name - the world is at his feet.

"I'm just enjoying doing what I'm doing, just going with the flow," Grint concludes. "It's important to me to have a lot of time out as when I finished the last Potter film, I just didn't want to do anything. So it's nice to have some free time to just do what you want to do."

  • Watch Sick Note on Sky One and NOW TV from Tuesday