'Fame does not last forever'

"On a film set, ideally," he states, eyes widening.

"I want to ruin someone's day. I want to have them suddenly go, 'Dan's just dropped dead in front of the camera; we have to get his double on'."

The prelude to this is pondering a life without acting.

"If someone told me tomorrow, 'You're never going back on a film set', I really wouldn't know what to do with the rest of my life.

"I've never not worked, so I don't know what my life looks like without regularly being on a film set. I think I'd go crazy."

The truth is, the 27-year-old, who was 11 when he was cast as Harry Potter, could easily never work again, having already amassed a fortune of £74 million, according to The Sunday Times Rich List.

It's an amount that's beyond comprehension for most people.

"It is for me!" exclaims Radcliffe, as we sit facing one another on a sofa.

"I don't really do anything with my money," he adds. "I'm very grateful for it, because having money means you don't have to worry about it, which is a very lovely freedom to have. It also gives me immense freedom career-wise."

Radcliffe, who's dating American actress Erin Darke, takes this freedom seriously.

"I feel you have a responsibility when you can be that choosy," says the actor, who remains an energetic presence, although the agitated tics so often described in the past seem to have subsided.

"For all the people who've followed my career, I want to give them something to be interested in, rather than them just watch me make loads of money on crap films for the rest of my life.

The seventh and final Potter film was released in 2011. Since then, he's appeared as a young physician in the dark TV comedy series A Young Doctor's Notebook, the haunted Arthur Kipps in the The Woman In Black, gay beat poet Allen Ginsberg in Kill Your Darlings, and a young man who finds horns sprouting from his skull in, aptly titled, Horns.

He's also earned acclaim on stage, in dramas such as The Cripple Of Inishmaan and the musical How To Succeed In Business Without Really Trying on Broadway.

"I think it's about doing things that are unexpected, and I love that I'm hard to pin down or categorise," he says of his eclectic choices.

"I've always felt very secure. The biggest risk people see me as having taken was doing Equus [the 2007 stage production co-starring the late Richard Griffiths], and that never felt like a risk at all, because I was just looking around at all the other people in the room thinking, 'Well, if I'm screwing up, then a lot of other amazing people are screwing up as well'."

The production required him to spend a large proportion of time on stage nude, which caused a bit of a furore among a public who could only associate him as the boy wizard.

"Well exactly - Harry Potter blinds horses and does a not very child-friendly play - which some people still brought 10-year-olds to," retorts Radcliffe, shaking his head in bewilderment.

The London-born actor, the son of a literary and casting agent, continues to surprise.

Today, he's promoting Imperium, in which he plays Nate Foster, an FBI agent who goes undercover as a neo-Nazi in order to expose a terrorist plot.

The role required him to shave his head, wear clothes emblazoned with the likes of 'White Power' and shout racist obscenities.

"When we filmed the rally riot scene, we only had one camera, so from a distance, people couldn't see it; they just saw a Ku Klux Klan rally walking through their town. We had people who were very angry and had to explain it was a film," he recalls.

"We also had someone wind down the window, beep their horn and give a 'white power' [salute], so it was a very weird scene to shoot."

But despite the "heavy topic", it was "a very fun movie to make".

As was Swiss Army Man, another new movie.

"It was maybe the most fun I ever had on set," remarks Radcliffe of the film, which prompted boos and walkouts when it screened at Sundance.

It tells the tale of a castaway called Hank (Paul Dano), who's about to take his own life when a very flatulent corpse named Manny (Radcliffe) washes up on the shore.

A friendship is formed, and the pair embark on an adventure together with the aim of returning Hank to the love of his life. Critics have lambasted the film for its 'puerile' scenes, but Radcliffe has nothing but praise for directors Dan Kwan and Daniel Scheinert, known collectively as 'Daniels'.

"I think they're playing with the idea there are certain things about being human that are universal, whether it's farting or getting erections or masturbation, or things that aren't bodily function-related, like being lonely or feeling different or weird," he offers.

"These are things we're all taught are slightly shameful, and the driving premise of the film is that shame is what keeps us from love and keeps from loving ourselves."

It's rare for people to speak about their profession with the gusto that Radcliffe does.

"It's because I'm one of the lucky few who f**king loves my job. Most people hate their job and it's something you have to get through. Whereas I used to feel I had to get through the bits of life in-between jobs, like I'm working to get back to the job."

He doesn't, he clarifies, feel like this any more.

"I've got much better at relaxing and enjoying down-time and hanging out with friends," says Radcliffe, who recently returned from two weeks in Greece after wrapping on Jungle, due for release next year.

He hopes to take some time off to write ("It's the thing I always want to do"), before appearing on stage again in London next year.

Does he ever crave anonymity?

"Sometimes," he admits, nodding. "Privacy is amazingly important, because that's where you grow.

"Ultimately, the hardest thing about growing up in the spotlight, it's not the easy access to drugs or the strange, sort of pandering world that you enter into, but the difficulty is trying to work out who you are while you're constantly coming up against a perception of yourself that everybody else already has," he adds.

"I think it's very important, especially when you become famous young, to work out who you are without fame and without that as part of your identity, because that will go.

"Fame does not last forever. For anyone."

:: Imperium is out now

:: Swiss Army Man is released on Friday, September 30