“I wanted to be a fireman,” says Jack Osbourne, looking at his hands. “Or an astronaut. Or a soldier. I still kinda do. Is it too late?”

That’s my Jack Osbourne epiphany moment. It’s not the funniest line of our interview (which is about snakes and spiders, and is a few paragraphs down), or the most emotionally revealing (which is a casual aside about his primary-school friends knowing that his dad had tried to strangle his mum), but it opens him up like a clam. Jack Osbourne may be an ex-alcoholic, a former child star and a reluctant prisoner of reality TV, but the man can do anything he wants. Anything in the world. He just needs to decide whether to pitch it at ITV2, Virgin1 or MTV.

Jack Osbourne speaks in the sort of tones Loyd Grossman might have used, were Grossman co-starring in Clueless alongside Alicia Silverstone. Most sentences sound as if they are questions, even if they are not, and many of them include the word “f***”. Normally in an interview, you’d leave that out. With Jack Osbourne, you just can’t. For example:

“I get offered a lot of f****** weird shows,” he says. “A drinking-game show? Oh, for f***’s sake. And there was this one – f***! – where they said, ‘Jack’s going to go around the world and get bitten by the most dangerous, venomous creatures on the planet! And administer the antidote right away!’ I mean… F***. Are you f****** kidding me? The Amazonian viper and the f****** fruit spider? Give me a gun with a bullet in it and I’ll just play f****** Russian roulette with you, for fun. Who the f*** does that?”

Not Jack f****** Osbourne?

“Are you f****** serious? Get stung by, you know, Japanese hornets? I mean, f***. They e-mailed it in. You should read it. It’s not bad enough that I have to go to Lagos, which is a f****** horrible place as it is. No, I have to get bit by some f****** spider which they have out there, too. F***! F*** that.”

You get the idea? Jack Osbourne is a f****** pro, f****** on your television since he was 15. That was when he emerged in The Osbournes as Ozzy and Sharon’s podgy, foul-mouthed, indolent and yet curiously insightful young son. If The Osbournes was The Simpsons, and Ozzy was Homer, Jack was Bart. Sister Kelly had the more colourful tantrums, but Jack was the soul. Over the coming weeks, he’ll be on our screens in two guises: in Celebrity Adrenaline Junkies, already under way on ITV2, and in Virgin1’s The Prisoner: X.

It is the latter which has brought us here, to a rooftop in Marylebone, London, to talk about reality television. On paper, it sounds like a production company’s practical joke. Take a handful of Britain’s most popular reality stars – Joe Pasquale, Will Mellor, etc – and chuck them into the harshest and most notorious jails on the planet. Osbourne got a week in the Shelby County Criminal Justice Centre in Memphis, Tennessee.

“I never was scared,” he says. “I felt uncomfortable, but never scared. There’s something about when you have a camera. These guys looked at it like it was their time. Their box to stand on and tell their plight. They weren’t interested in who I was. The only guy who recognised me was one of the sheriffs.” Most of all, what he remembers was the smell. “People. Kinda masked with detergent. Like, cream and… hospitals. F****** horrible.”

To call Jack Osbourne somebody from reality TV is a bit like calling the Princess Royal somebody who launches the odd ship. Jack is not some Big Brother alumnus. He is the youngest member of a proper showbiz family firm, branded by his father and managed by his mother. It was Sharon who negotiated the original show with MTV after the channel approached her with a view to getting her children to present. Famously, the children were asked whether they fancied being in it. Aimee, Sharon and Ozzy’s eldest daughter, declined. Jack and Kelly opted in. Years on, you get the impression that Jack, at least, hadn’t really grasped what he was opting in to.

“I didn’t… I didn’t comprehend at the time that what was being filmed would be on TV,” says Jack now, frowning. “You look back, and you think about all the shit that people were saying about us. About kids. It’s f***** up to judge kids. You don’t know who you want to be, who you even are, and then all of a sudden you’re thrown on to one of the bigger shows of the time…”

By the second series, he had stopped watching it. That was the season when his mother was diagnosed with cancer. “It made me resentful. Of the family. Of everything. And then I kinda had to block it out. People were coming up and saying stuff to me in the street, and I’d be like, ‘How… How… How do you even know about that? I didn’t even know that was on the show!’ So I started drinking and doing other stuff. Maybe it was bound to happen. But it had a lot to do with the way I left school when I did and started… f***… started getting paid to stay at home. It kinda just threw petrol on the fire.” By the third series, Sharon was in remission, Kelly was a pop star, and Jack was drunk, stoned and brattish.

Explore Celebrity

• People

Times recommends

• Tewkesbury: when the memories flood back
• How I lost the gift of joy for a while
• The A-Z of winter style rules