Radcliffe’s father, Alan Radcliffe, a former literary agent who left his job to chaperon his son on the set, had a phrase he used if Radcliffe ever did appear put upon: “You’re not down in the mines.” It was shorthand for: You are incredibly lucky, and you are being well compensated, and your worst day, in many ways, is better than most people’s best. After Potter was over and Radcliffe, then 21, went to shoot “The Woman in Black,” the first time he wouldn’t be accompanied by a parent, his father wrote him a letter. “On a film set there’s always going to be somebody who’s going to be causing a delay,” Radcliffe recalls the letter saying. “Try and make sure it’s never you.” For 11 years, Radcliffe had done just that, every day, by all accounts; why spell it out? “Constant vigilance,” Radcliffe said, “is kind of our motto.”

Radcliffe’s upbringing during the Potter years was at once cloistered and uncensored. His father was on the set every day, providing an unusual amount of oversight in the strange, charmed halls of the drafty, converted aircraft hangar where the movies were shot. But all around him, especially as he became a teenager, the crew and cast were swearing, changing in front of him and regaling him with tales of boozy revels. Although he and Watson and Grint shared an intense experience and are friendly enough, they have barely seen one another since the last film. Radcliffe’s closest friends were always among the crew, people who “were either much older than me and had kids or lived outside of London,” he said. “I didn’t have that normal teenage period when you build up your friends in your area and you have a social circle.” After what was usually a long day on set (which included three to five hours of tutoring), Radcliffe went home and found anarchy where he could: he played the Sex Pistols and the New York Dolls, his childhood bedroom a refuge of rebellion. The weekends were for homework; he almost never went out with peers, self-conscious about having to reassure his parents about his safety or worried about imposing the hassles of celebrity on his friends.

At 17, Radcliffe moved out on his own, something he had been wanting to do for a long time. “Because of the life I’ve had, I’d grown up quicker than most people,” he said. “I felt like I was entitled.” He was also tired of being watched. “I can be honest about this now, because I know my parents know — but I wanted to smoke,” he said. “I was hiding it like a fiend.”

Looking back, he thinks he was too young to have been on his own. “Because when I was unhappy in any way, it made it too easy for me to hide it,” he told me one afternoon over lunch in the New York’s West Village. “I’d done ‘Equus,’ which had gone so well,” he said, “but I still couldn’t get rid of that committee of voices in my head saying that you’re going to fail.” He continued: “I think there was a part in the back of my head that was going: This is all going to end. And you’re going to be left in this nice apartment. Just living here. And being reminded of what you did in your teenage years for the rest of your life.” David Thewlis, who played Professor Lupin in the Potter films, once said that even when Radcliffe was young he would “joke that he’d be in rehab by the time he was 18, and by 27 he’d be hosting a game show called ‘It’s Wizards!’ ”

Not long after Radcliffe moved out on his own, he started drinking. By his own description, this was not casual drinking at parties, but every-night drinking, heavy drinking, drinking to the point of making a scene and then blacking out. “I became a nuisance,” he has said. “I became the person in the group who has to be looked after.” He drank in local bars and eventually alone, because he was too embarrassed to go back to the bars where he had been so drunk on nights past. In August 2010, when he was 21, after awakening from a blackout, bruised and unable to account for the previous eight hours, he decided to stop drinking. He hadn’t talked publicly about the extent of his problem and was worried that he would rise one morning to find photos of his past exploits plastered on the front page of a tabloid. So in 2011, he decided to speak openly about his drinking. Having spent so many years protecting the image of Harry Potter, he felt unknown by the same public that considered him an intimate part of their childhoods.

“I wanted to close the gap between the real me, what was going on in me, and the person that people perceived,” he told me. He talked to reporters about drunk-dialing old girlfriends, the strain drinking put on his relationship with his parents and even, most sensationally, the times he showed up, still drunk, on the set of “Harry Potter.” David Yates said he was not aware of Radcliffe’s drinking, which Radcliffe believes is true; he had carefully hidden his problem from colleagues and many close friends. “I’m not somebody who likes worrying people,” he told me. “So if I know I’m a worrying drinker, would I ever drink in front of people that I would worry?”

The tension between a protected and overexposed life still exists for Radcliffe. When he’s working in New York, he shares his apartment downtown with his personal assistant, Spencer Soloman, a 38-year-old former dancer and camera man, who became close with Radcliffe’s family when he was teaching the actor to dance for “How to Succeed.” Soloman and Sam function, in some ways, like older brothers: fun, but responsible and organized. Soloman plans Radcliffe’s schedule and talks to his father, agents and publicists two or three times a day; he might tell Radcliffe when he needs to shave for a photo shoot or search their apartment for a garment that his stylist wants him to wear for some occasion.