"Want to see the grimmest apartment ever?"

This is Justin Theroux, failing miserably as a Realtor as he leads me southward on Lafayette Street into the slanted afternoon sunlight. But Theroux isn't selling. He's guiding, and this stop—a wide charmless slab of a building at the juncture of Bleecker and Mulberry streets so gray and dispiriting it seems to cloud over the early-summer day—is the first on a loose and spontaneous tour of his downtown-Manhattan haunts, both past and present. This one falls squarely into the former category, dating back to the early 1990s, when Theroux was cobbling together a sort-of living by hauling Sheetrock, painting murals in dance clubs, bartending, and, whenever possible, acting. He crouches on the sidewalk and points to a narrow slot of glass at ground level. "That was my window," he says, inflecting the last word to denote that, as windows go, this one really doesn't qualify. "And by the way, what you're seeing there, that's also the top of the ceiling." It was a cut-rate basement hovel of an apartment, he explains, wedged so close to the boilers that "the temperature went up to 105 at night, and in the dead of winter you'd have to run the air conditioner to keep cool." Trains coursing in and out of the nearby subway station rattled the walls every few minutes. "It was fucking horrible," he says. "I think I lasted there a year. It was a long time. Total dark night of the soul."

New Yorkers love trading these early-apartment horror stories—the bathtub in the kitchen, the drug dealer down the hall, the rat leaping from a box of Lucky Charms—and Theroux, dark night of the soul notwithstanding, is no different. That's because—despite his Washington, D.C., upbringing and the new nest he's currently feathering out in Los Angeles with his fiancée, Jennifer Aniston—Theroux is a man who seems incapable, as several of his friends say, of existing anywhere else, his lungs unfit to breathe any other variety of smog. "New York is so embedded in him," says his good friend John Krasinski. "It's in the fabric of what makes him him. He's the quintessential New Yorker." And while he "never looks set-designed," as Krasinski says of his friend's style, Theroux's clothing often announces his citizenship, as it does today: black leather boots, cuffed black jeans, a faded gray pocket T-shirt layered atop a white T-shirt, a black baseball cap dangling from a rear belt loop—the uniform of someone who might've lugged a beat-up guitar case into CBGB back in its heyday. At 42 (he turns 43 this month), even his physical attributes seem citified: the licorice-colored hair bequeathed to him by his Italian ancestors, the way his widow's peak and arching eyebrow lend his expressions a noirish cast.

Almost within sight of the old CBGB space, now occupied by a John Varvatos store, is Von, the Bleecker Street bar that was Theroux's final place of employment before his acting career took off. Besides tending bar, he contributed artwork, including the stenciled rendition of the bar's name that remains on the window. "That was me," he says, like a graffiti artist identifying his tag. Also in Theroux's "that was me" category: the hipster cheap-beer renaissance. "Yeah, I still take credit for the idea of selling cheap beer for lots of money," he says. "It was a whim. I was like, 'Wouldn't it be funny to sell Schlitz and Pabst Blue Ribbon up there with all the great wheat beers?' We added every horrible beer we could think of and charged four bucks for them. Those beers ended up outselling Chimay, everything. So that might've been me that started that and gave everyone the squirts. You're welcome, Pabst." ("I'll bet if we dug deep enough," says Krasinski of this claim, "we'd find that he was somehow behind whatever we think is cool.")

The bar isn't open yet, so we hike past another of Theroux's old apartments to where Bleecker smacks into Bowery. "This was kinda my corner for the longest time," he reflects. "Ten years. I never thought I'd see the day when this was a fucking . . ." But he stops himself here, wary of the wistful lament, the downtowner's cliché. "I've never been particularly nostalgic," he explains. "I'm not sure whether nostalgia is a weakness or a strength. I lean toward weakness, because it takes you out of experiencing what's cool if you're constantly the guy who's like, 'Oh, this place used to be amazing, and now it's shit.' Because that's not true. It's the same reason I don't keep pictures of friends and family around. I don't need to be reminded that I have friends and family."

Yet nostalgia is wired into every New Yorker's psyche. As the novelist Colson Whitehead once wrote: "You are a New Yorker when what was there before is more real and solid than what is here now." It's part of the way New Yorkers track their relationship with the city. And every transplant, like a religious convert, can recall that moment when the city's neon spirit first moved him, Theroux included. "I sound like a kid saying this, but I think I saw a movie when I was young, maybe Flashdance or Rocky something, and I remember thinking, I gotta fucking get there," he says. "It actually wasn't even New York in those movies, but it ended up becoming New York." His first visit alone came at the tender age of 14, and he developed an instant teenage crush on the city. "It wasn't even the old thing about if you can make it there, you can make it anywhere. It had nothing to do with that. You just felt really free. It felt like no one was going to judge you. You could disappear here and be more yourself than you'd ever been."

The young man who came to New York City to disappear, however, has, in recent times, done anything but. There is his rippled, tattooed back towering over Times Square on a three-story-tall poster advertising his leading role on the acclaimed new HBO series The Leftovers, the latest project from the Lost co-creator Damon Lindelof, which premiered in June. And thanks to his two-year engagement to Aniston, there is Theroux on the covers of half the tabloids greeting people in supermarket checkout lines, his every coffee break documented by paparazzi and then scrutinized for any repercussions that said coffee break might be having on his upcoming marriage to America's Sweetheart.

If there's dissonance here, so be it. Justin Theroux is comfortable with dissonance. The slash that divides his identities in the public imagination—the difficult-to-place, chameleonic character actor–slash–Kimye-level tabloid fixture—is just one more slash in a life that's lacerated with them. He's collected and curated slashes, in some sense, in much the same way he once collected Art Deco cabinet knobs and Black Label beer cans and wooden tongue depressors and human teeth. His job title, for starters, is rife with them: actor-slash-writer-slash-director. The Hollywood hyphenate is hardly unique, sure, but dig a little deeper into Theroux's résumé and peculiar sub-slashes start turning up: He's one of the director David Lynch's favorite ensemble players, an actor capable of conveying intellectual grit and weirdo gravitas with a single raised eyebrow, and yet—slash—he's often the joker lurking at the fringes of Paul Rudd rom-coms. He directed a quiet and cool art-house movie, Dedication, just a few years before writing the screenplay for Iron Man 2. He's a downtown-Manhattan diehard, yeah, but he's also "bicoastal," as he defines himself now, with a massive home in Bel Air where he and Aniston tend to a flock of chickens and three dogs.