From the time he arrived in New York, after graduating from Bennington College in 1993, he has been cultivating these slashes. Most would-be artists arrive in the big city with a single dream. Slashes are usually what happens when reality contaminates the dream—the actor-slash-waiter, the model-slash-bartender, that sort of thing. But Theroux came to New York City pre-hyphenated, bearing dual degrees in visual arts and drama to back his ambition to become a painter-slash-actor. "I had these two terrible careers," he says. "I figured I'd throw both of them at the wall, and maybe one of them would sloppily stick." To varying degrees, both did: For a time, he was a sort of artist-in-residence at the Palladium, the iconic Steve Rubell–Ian Schrager club on 14th Street that was the locus of nineties nightlife. "They'd bring these huge pieces of paper that you tear down, and I'd draw and paint while the club was happening, surrounded by women dancing in cages," he recalls. "It was like a stage, like an artist on display, and they'd give me these huge fat markers and I'd draw something big. It was just part of that club-life circus back then." On the acting side, there was off-Broadway theater, followed by roles in the 1996 film I Shot Andy Warhol and a smattering of Sex and the City episodes and even an appearance on Ally McBeal. "The seesaw sort of tilted toward acting," he says. "But I was happy to do either of them."

The word versatile is often used to praise actors, but it's typically applied to their performance abilities, not to their temperaments or biographies. "J.T. is a renaissance man in the truest way," says Robert Downey Jr., who befriended Theroux when he was a writer on Tropic Thunder and later brought him on to write the Iron Man sequel. "He can act, he can draw, write, direct, curate oddball collections—the whole deal." (Theroux's longtime friend Amy Sedaris would add one more skill to Downey's list: "He's a very good crafter. He once made me a bottle-cap catcher out of a coconut," which she says will impress anyone who's ever tried crafting with coconuts.) Lynch, who turbocharged Theroux's acting career in 2001 when he cast him in Mulholland Drive, says Theroux's multifacetedness is part of what makes him such "a modern man." "The boundaries are breaking down," Lynch says, "and everyone is doing many more things."

"It's only one lifetime that you have," is how Theroux explains it as we dip into an antiques store on 22nd Street. "So you have to, in a weird way, keep pivoting on whatever it is you're doing. Follow whatever it is that's keeping your interest." Though painting has fallen by the wayside, "I'm sure if everything else went away and I picked up a sketchbook again, it'd rekindle something," he says. "I think I'm a little bit—I don't want to say A.D.D., I don't think I'm that, but I can get bored with something quickly."

He's at the store, Mantiques Modern, to check on an item he's been eyeing for years: a three-foot-wide wooden replica of the famed Lion of Lucerne rock sculpture in Switzerland. "It's amazing, isn't it?" he says, staring at the sculpture through its glass case. "There's something really noble about it." In leaner times, Theroux used to scavenge his antiques from Dumpsters. "I used to garbage-pick," he says. "It sounds much more disgusting than it is. You learn a lot about society by seeing what's thrown away. During the period I was doing that, a lot of great stuff was being tossed. What do they call that now, 'Ironweed chic'? Edison bulbs and all that shit. This was stuff I was finding for free 15, 20 years ago." The shop's owner tells Theroux the price of the lion: $9,500. "Oh, it's down," Theroux says. "When I came in a few years ago, I think it was 14." The owner offers to remove it from the case for Theroux to examine, but he passes. "Oh, I've pulled it out before," he says, and sighs. "It's one of those things I'll just keep thinking about until I regret when it's gone."

Of all Theroux's pursuits, the writing—which came last and which includes, besides Iron Man 2 and Tropic Thunder, the screenplays for Rock of Ages and the forthcoming Zoolander 2—would seem the most ordained. His mother, Phyllis Theroux, is an essayist and a memoirist. One of his uncles is Paul Theroux, widely considered one of the greatest travel writers of the 20th century; another uncle is the novelist Alexander Theroux; and yet another is the translator Peter Theroux. His cousins are the celebrated British journalist Louis Theroux and the novelist Marcel Theroux. Art Buchwald and Kurt Vonnegut were some of the frequent guests at the family dinner table. (Theroux's father, a corporate lawyer, exited the household in a divorce when Justin was in grade school.) Yet Theroux credits that heady, salonlike atmosphere with generating his sense of humor rather than any literary inclinations. "I wasn't good at those conversations," he says, which may have been partly because of the dyslexia he struggled with as a child. "I mean, there were dinner-table conversations with my uncle Paul when we'd start arguing about where the Crusades began. And at a certain point, I had nothing to add to that. Actually, from the very beginning, I had nothing to add to that. So you put your hand under your armpit and make a fart joke and you've done just as well in a way."

"He's incredibly funny and silly," says Sedaris, who recalls buzzing Theroux on a visit to his apartment and then hearing his voice thunder over the intercom, "Amy, I'm not in love with you anymore—stop bothering me," Sedaris standing mortified on the sidewalk as passersby withered her with pitying glances. "But he's not annoying about it, like some people can be. He's not always on." What friends and directors tend to circle around is Theroux's intelligence—offscreen and on. "Justin always has that mind going," Lynch says. "He shows thinking on the screen. You can see his mind working." Lindelof, unprompted, echoes that sentiment almost identically: "There's a fierce intelligence to his acting. You can always see him thinking."

Yet Theroux wasn't an obvious choice to lead a dark, theologically knotty cable series, in which he plays a small-town police chief grappling with the aftermath of a Rapture-like event in which 2 percent of the world's population has disappeared into thin air, like popped soap bubbles. "He's a little unknown—a bit of a cipher," Lindelof says. "That's exciting to me. There's a where-did-this-guy-come-from aspect to Justin that's really fascinating." That cipher quality stems from the roles Theroux's taken over the years, which, despite his dramatic turns with Lynch and others, have tended toward the comic end. That's another enigma about Theroux that Lindelof savors: "Very few actors can move between a comedy and a drama space. It's not always clear which is the true Justin. Is he the goofball with his shirt off playing guitar in Wanderlust, or is he the dramatic actor? Which suit goes on more comfortably?"

"There are certain actors who can grab a woman by the back of her hair and plant a deep one on her and say something like 'We gotta save the world,' and that's not me," Theroux says, excluding at least one cinematic suit. "I can't not be outside of my body, making jokes about that dialogue. Some people do it really well and effectively because they have the charisma to pull that off. I just know that I don't."

"I don't know that that classic leading man exists anymore," says Tom Perrotta, who wrote the 2011 novel The Leftovers and co-created the series with Lindelof. "Justin is certainly handsome enough, but there's an emotional awkwardness that's pretty fascinating about him. He has this combination of outward strength and inward bewilderment." That sense of bewilderment was crucial for a series about, well, a bewildering catastrophe and all its debilitating strains, and Perrotta and Lindelof suspect, with obvious hopes attached, that the series could provide Theroux with the defining character that's so far eluded him. "I think this role is going to cement him in the public imagination," Perrotta says.