Why Daniel Radcliffe won't be drinking cider with Rosie

In a New York diner, Daniel Radcliffe is shrinking in his seat.

It is May 2011, and the titular star of Harry Potter, the biggest franchise in movie history (the global box-office gross of the eight films is $7.7 billion) is starring in a Broadway musical, How to Succeed in Business Without Really Trying.

He's a regular here, and the waitress - a gum-snapping, Bronx-accented firecracker - has greeted him like a long-lost friend.

At that meeting last spring I was nearing the end of a year shadowing Radcliffe for a story for a men's magazine. As part of the fitness regime required to take the lead, eight times a week, in an all-singing, all-dancing, totally gruelling theatrical production, Radcliffe was eating healthily and working out.

So he wasn't eating. Nor, it transpired, was he drinking. At the beginning of our year together, in September 2010, he'd told me he intended to stay off the booze for the duration of the upcoming 12-week shoot on The Woman in Black, the horror film that would be his first post-Potter project.

But it turned out that Radcliffe's teetotality wasn't simply a function of the actor wanting to give his all to the modestly budgeted chiller. He had to lose the alcohol before the alcohol lost him.

I was reminded of an encounter in the upstairs bar of the Ivy in 2009, when Radcliffe, then 19 and no longer chaperoned everywhere, was off the leash and on the lash. As I sipped lager, Radcliffe had necked whiskey sours. And double bourbons. And a fistful of fags. Oh, go on then, tequilas too. "I love tequila," Radcliffe had said, positively licking his lips. "It's one of those things like Jägermeister, where you get a certain type of drunk off it." Thereafter, I confess, my memory is hazy.

Blackouts, he admitted, had become commonplace. I told him that the month before that encounter an acquaintance had seen Radcliffe acting rather, shall we say, lively in the street outside Sketch. He cringed. "That's the reason I stopped - every time I do something like that, it's the next day where everybody goes, 'So, do you remember this  ?'" So now it was sobriety all the way. He was much happier for it. "I'm actually enjoying the fact that I can have a relationship with my girlfriend where I'm not f*****g up totally all the time."

When that year-in-the-making feature appeared last summer - just before the release of the final Potter film, The Deathly Hallows Part 2 - the "revelations" of Radcliffe's drinking problem generated headlines around the world. "Harry Blotto" sums up the tenor of much of the coverage.

When I caught up with him at the Soho Hotel last month, I asked Radcliffe, now 22, with a personal bank balance of some £48 million, how that media alco-storm had been for him. He cheerfully admitted that he hadn't imagined the news would generate quite the hoo-ha that it did. Mostly, though, true to level-headed form, he hadn't been bothered by it.

"The only thing that pissed me off was that occasionally you'd hear people saying stupid things. There was a woman on CNN who made some really crass remark like: 'Oh, it's funny that he's suddenly decided to talk about it now his film's coming out!' And I was just thinking: 'Oh yeah, 'cause Harry Potter eight, the last film, is so desperate for publicity that I thought I'd come out as an alcoholic!' That made no sense to me."

Now, having worked through his teenage party-boy wobble, Dan Radcliffe is even more dedicated to The Work. And also, he admits, to his girlfriend. During the course of our year together he had artfully, chivalrously and sensibly concealed her identity. But she has subsequently been outed as Rosie Coker, whom he first met when she was a member of the production crew on the Potter films. Again, he's not bothered that his private life - bits of it anyway - is now out there.