When you meet Tom Hardy, it’s difficult to know what to expect. Or rather, whom.

His latest part, in the title role of BBC Two’s adaptation of Stuart: A Life Backwards, Alexander Masters’s bestselling biography of a homeless alcoholic, is the latest in a series of remarkable transformations for the 30-year-old actor, whose screen roles so far include the ghost of a psychopathic murderer in Cape Wrath, a flouncing dandy in The Virgin Queen, and a mad alien in Star Trek: Nemesis.

Ironically, his versatility may be the reason he is not yet a household name. But his extraordinary appearance in Stuart will surely, finally, seal his reputation as a heavyweight contender. A dazzlingly accomplished performance, it is already attracting predictions of Bafta glory. It has also led to comparisons between Hardy and Ewan McGregor, whose career vaulted into the major league after his appearance in Trainspotting, a role that involved a similarly startling physical transformation. Masters’s highly acclaimed, much-loved 2005 biography of Stuart Shorter, a violent, homeless, charming drug [[ addict whose brief, grim life ended in 2002 when he stepped in front of a train, was a surprise nonfiction bestseller.

The BBC Two adaptation is scripted by Masters, who met Shorter while working as a volunteer at a homeless charity in Cambridge, and it is an exceptional achievement: a sad, sweet, furious, horrifying, warm-hearted, heartbreaking and hilarious account of a difficult life altered – and, briefly, lifted – by a wholly unlikely friendship (the passionate, relentlessly self-analytical and occasionally misguided Masters is wonderfully played by Benedict Cumberbatch).

“I love Stuart,” Hardy says. “I love him. I feel for him, his pain, d’you know what I mean?” This empathy is at the core of Hardy’s portrayal of Stuart, a man whose heroin addiction, alcoholism and violence are accompanied by warmth, self-awareness, humour and an abiding appreciation of the restorative benefits of Shake ’n’ Vac (“it’s good stuff, that”).

Such is the accuracy and affection with which the actor captures the contradictions at the heart of this self-proclaimed “sociopathic street raconteur” – the hunched, muscular dystrophy-affected shoulders, the emaciated heroin shuffle (Hardy lost two stone for the role), the pomposity, the politeness, the sudden, shocking bursts of brutality and the incongruous, touchingly fussy desire for order – that you don’t know whether to congratulate him or hand him a fiver for a bottle of White Lightning.

“There’s more pain in him than there is in me,” reflects Hardy, “and there’s a gratitude there in some ways, because I think, ‘Well at least I’m not in as much pain as this poor guy.’ I don’t want to make it sound sentimental, though. Alexander has written something that sings. I fell in love with the piece. I could hear the music of it.”

Masters is happy to return the compliment. “It’s a stunning performance. Tom seems to have absorbed the character completely,” says the writer. “On and off set, he lived the character. I’m astonished at how well he managed to portray Stuart’s humour. He was an extremely funny man with so many insights. It took me a long time to get the tone of the TV production right, to get the right balance between the pathos and the humour. But Tom has managed to capture the complexity beautifully. That’s an incredibly difficult thing to do, I think.”

Hardy’s affinity with Stuart was no less personal. “I had a lot in common with Stuart, with the exception of a couple of very serious experiences – specifically the child abuse he suffered, and the muscular dystrophy. Everything else I have full comprehension of,” he says. “I could relate to the drugs and his attitude at points, too – d’you know what I mean? There were a lot of similarities between us. I thought ‘Oh, oh, oh [he bounces up and down on the sofa in excitement, his trainers pitter-pattering on the carpet], I can bring something to this!’ It came from my own baggage.”

This “baggage” is, for a self-confessed “nice middle-class white boy from East Sheen”, quite surprising. A former “dabbler” in drugs and heavy drinker (he’s been clean for two years), Hardy was arrested at 15, having been caught joyriding in a stolen Mercedes. Since then, the boarding school-educated former model (he had a contract, briefly, with Models One) has fought a continual battle against a predilection for “dangerous and dodgy things. I have to constantly steer myself away from them.” Such behaviour stems, he says, from an innate sense of “not being good enough”. Acting has been both a lifeline and a curse, allowing him to “keep my mind off myself” while feeding his perfectionism.

“I don’t like quiet. I don’t like having to sit with myself. But no matter where you go, there you are. I hate being on my own. The lights are always on, if you see what I mean. I’m learning to sit still but it’s hard. Really hard.”

While expressing “total excitement” over his appearance in Guy Ritchie’s forthcoming London underworld film, RocknRolla,Hardy’s most pressing concern is, as always, his self-doubt.

“If I could get rid of that, I would feel a lot better. I’m trying. Playing Stuart helped. He taught me about me, about what I’m capable of, about fears and embarrassments. About not giving a s*** what anyone thinks of you. These are small issues for some people but for me they’re huge.”

And what of the future: the prospect of further weighty, character-driven roles, the comparisons with Ewan McGregor, the lure of Hollywood, the acclaim?

“Stuart was the best role I’ve ever had – probably ever will have,” he says, solemnly. “I could stop now, technically. I’m not going to, because I love the craft, I love playing, I love acting. But that is as good as it gets, for me. So I can’t say what will happen next. Even though I know there’s no such thing as perfection, I’m still looking for it. It drives me. But I have a fear of success and a fear of failure. Self-sabotage is in my make-up. It can be debilitating. I can’t standbeing in my head, that’s why I have to get out of it. That’s where the drugs and drink came in. I don’t do any of that any more, though. That’s why I have to act.” He pauses, frowning. “D’you know what I mean?”

Stuart: A Life Backwards, Sun, BBC Two, 9pm

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