I was 10 when everything changed," says Rupert Grint. "And it did change everything."

The man who was schoolboy wizard Ron Weasley in the Harry Potter films is sitting opposite me talking about life beyond Hogwarts. The franchise (completed in 2011) made him rich, but he has a strange relationship with money - "I only have one pair of shoes".

It also made him famous, but he has an odd relationship with fame, too - "Occasionally, if I walk quickly and wear my cap, I can kind of blend in a little bit." And it made him feel different from the rest of his generation - "I do feel like I'm the only person who's not on Twitter." Grint is 29.

His decision not to be on Twitter, he says, is not a protest against modern life, it's just that he wouldn't know how to project himself because he likes to protect his privacy.

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He still gets asked to pose for selfies almost every day by Potter fans, but his desire to stay out of the spotlight is so well known that his attendance at a London theatre performance of Harry Potter and the Cursed Child was reported in the media as a "rare Rupert Grint sighting".

"My presence was noted," he says, with irony, when I mention it. "Seeing someone else's interpretation of Ron is like an out-of-body experience," he adds.

"It's very surreal. I feel like there's so much of me in Ron, it's hard for me to decipher who is what. It was quite emotional in a weird way."

Grint is boyish and diffident, softly spoken. He may have found it hard to detach himself from Weasley, but since Potter he has been pretty busy.

He's made some interestingly offbeat independent films, such as the "Stanley Kubrick faked the moon landings" comedy Moonwalkers (2015); he's acted on stage in London and on Broadway; and he produced and starred in the television spin-off of Guy Ritchie's Snatch - unloved by the critics but already commissioned for a second series.

From left to right: Emma Watson as Hermione Granger, Daniel Radcliffe as Harry Potter and Rupert Grint as Ron Weasley. Photo / Supplied
From left to right: Emma Watson as Hermione Granger, Daniel Radcliffe as Harry Potter and Rupert Grint as Ron Weasley. Photo / Supplied

Now he's appearing in a Sky One comedy, Sick Note, as Daniel, a waster who is diagnosed with terminal cancer.

On Sick Note, Ruper Grint stars as a waster who is diagnosed with terminal cancer. Sky One / Now TV

Suddenly his boss, his parents, even his girlfriend start being nice to him. Then he finds out that it was all a mistake but, rather than lose the benefits of the misdiagnosis, he decides to keep his mouth shut.

"I'm drawn to the dark side of humanity, and finding humour in that is something that interests me," says Grint. "I love the way the lie spirals out of control and other characters become tangled in it. Daniel is tricky to play, though - he's not the most likeable character."

To his credit, Grint makes Daniel likeable and funny, although he insists he's the straight man of the piece. He's acting opposite Nick Frost, who he says is relentless in his compulsion to make people laugh.

Just before a take, Frost would be looking at his phone, then come out with a line like, "Oh my god, Helen Mirren's died, poor Helen". "And then it was 'Action!'," says Grint, "and I wouldn't be able to get it out of my head."

He seems comfortable playing comedy. But is he the sort of person who lies in certain situations? "Yeah, I do it without even thinking. I'm not one of these people who say it like it is. That can be almost rude. I lie fairly often, not big lies, mainly getting out of social events I don't want to go to."

He's the oldest of five children, born in Essex but raised in Hertfordshire where his father sold Formula One memorabilia.

"It was a normal childhood in a happy home," he says. "I was always quite a shy kid - acting was my way of dealing with that, I think, pretending to be someone else."

He performed in school plays and for a local theatre group. At school, his ginger hair attracted the usual playground abuse: "I'd always get called, like, 'ginger pubes'. It's never really affected me too deeply, but I think that [prejudice] is changing now because of people like Ed Sheeran."

On Sick Note, Ruper Grint stars as a waster who is diagnosed with terminal cancer. Sky One / Now TV

Is it true that he gets mistaken for the singer? "Not so much now, but it did happen quite a lot," he says. "I even met Leo Sayer at Formula One and he was asking me about my music. I just played along with it."

He's a big music fan - "I've always been a bit of an outsider; I never really know what's going on in the charts. Although if I'm honest, I'd rather have been in a band [than be an actor]."

Grint has played a guitarist - in the 2013 film CBGB, about the famous New York club that launched the Ramones, Talking Heads and Blondie. It starred Alan Rickman as the club's owner Hilly Kristal.

When Rickman died last year, he joined a small but growing list of British acting greats and Potter stars who have passed away.

That must be hard to get used to? "It is. It happened very early on with Richard Harris." (Harris played Albus Dumbledore in the first two films.)

"It's odd. I can't claim to know them very well but it does feel like grieving an uncle, because we grew up on those sets and the people around us were part of the family."

When he says he grew up on those sets, he means it literally. "We had to do a minimum of three hours a day education and then go straight on set, but that could be broken up, so we'd be on set for a few hours and then have 15 minutes of school. They built a school in the studios because there were so many kids."

Was it a very chaperoned environment? "It felt very free, actually. We had our own playground mentality, and there were the same kinds of social cliques you'd find at school."

Who does he keep in touch with from the films? "Quite a few. I see Tom Felton [Draco Malfoy] now and again, and the twins [James and Oliver Phelps, who played Ron's big brothers Fred and George Weasley].

"We spent so much time together - it was such an intimate process, especially for me, Dan and Emma. It was every day for about 10 years.

"Everyone assumes we'll see each other all the time socially, but it's not really the case. We'll always have quite a strong bond, though, because it was a huge part of our lives."

He's aware of his post-Potter career being compared with the starrier ones of Radcliffe and Watson, but it's not something he thinks about. "It's fun to see them doing new things," he says.

His wealth is believed to be about £25 million ($47m). He's invested part of it in property, which his father helps him to manage. He did go through a phase of buying unusual vehicles, including a hovercraft, but that hasn't been used for a while, he says.

"There was a time when I was turning into a weird, Michael Jackson-type figure - just in terms of collecting weird things, but I think I've grown out of that a little bit," he says.

Grint has been through one of the most extraordinary childhoods imaginable. But for all the money and adulation that came with it, he seems a reluctant celebrity who hasn't made peace with his fame. The ghost of Ron Weasley hangs over him, and it has made him unsure of himself.

Even his concept of age is "a bit hazy", he notes, "I was 29 this year and ... I still feel like a teenager. I don't know when that will change. I can't imagine ever playing a father role in a film."

Sick Note S1 will screen on Thursday 23 November at 7.30pm on SKY BOX SETS.

- Daily Mail Telegraph