Radcliffe is also starring in “Horns,” a fall fantasy film he plugged last week at Comic-Con, and will make a cameo in the new Judd Apatow comedy “Trainwreck,” in which he plays himself in a movie-within-a-movie called “The Dog Walker.” Apatow pitched Radcliffe the idea in his Broadway dressing room, and he was game. “I’ve never done a movie without a script in my life,” he says.

Growing up, Daniel Radcliffe always thought Harry Potter would die at the end of J.K. Rowling’s books. “Because of the prophecy with Lord Voldemort,” Radcliffe says on a recent afternoon in New York, between cigarette puffs. “I thought, ‘How is she going to get out of that one?’ ” He finally worked up the courage to ask the bestselling author when she came to see him in the London production of “Equus” in 2007. “I was happy to be proven wrong,” Radcliffe says. “For an actor, what more can you wish for? You get a death scene — and then you get more screen time.”

This month, the questions rose to fever pitch after Rowling published a short story with a graying Potter. “Every time she writes something, I’m like, ‘Oh no!’ ” Radcliffe admits.

“I cannot envisage a set of circumstances where I would play him again,” says Radcliffe, who, at the end of a lengthy response about closing the door on Potter, offers a tiny opening: “Maybe there’s a chance that things go so well, I can go back.” For now, he’s rebuffed offers from Universal Studios to appear at “Harry Potter” events timed to their amusement park openings. “I felt like I had to draw a line,” Radcliffe says. “I’m always going to be incredibly proud of those films, but there comes a point where it feels odd to me to put on the robe.”

“I would also love to direct,” Radcliffe says, although he acknowledges that might be a few years off. “It’s something I feel like I could be good at, because I listen to people, and I love working with film crews. I’ve been able to watch a lot of directors, and cherry-pick what I think they do well. The mistake I see even really good directors make is that they assume they are the only creative person on set.” Radcliffe insists he’d be more democratic, and value everybody’s opinion.

“I didn’t want to go to Comic-Con without actually getting onto the convention floor. I’d prepared by buying a Spider-Man costume a few weeks ago. And what I’ve learned is that kids just love Spider-Man. A lot of people asked for photos with me as Spider-Man. It’s great to be able to put on a mask and interact with people who don’t know who I really am—I spoke in an American accent the whole time. I felt like a spy about to be found out at any moment.”

“I did a live interview with MTV and was tested on my nerd knowledge, and I failed the whole thing miserably. [Josh Horowitz] asked me to complete this phrase: ‘A Lannister always…’ and I said, ‘keeps his hat on.’ I don’t watch Game of Thrones! I was a huge comic-book fan growing up—Spider-Man, Daredevil, and The Flash. But I really must catch up on TV.”

“What’s really wonderful about Comic-Con is just how enthusiastic the crowd is. There’s an atmosphere where you can just be a big fan unabashedly and without apology. I thought I’d be more nervous going into the Hall H panel, but I was with [Horns director] Alexandre Aja and [book author] Joe Hill and they talk so well. Our moderator mentioned that it was my birthday a couple days earlier, and I said I’d always wondered what 6,500 people singing ‘Happy Birthday’ would sound like. And then they all sang. It was great. My life is made up of weird and amazing moments I’ll never forget, and that’s definitely been added.”