HOLLYWOOD—Rob Weiss, a writer and executive producer of the popular HBO series Entourage and the inspiration for its bombastic director character Billy Walsh, was sitting outside at the Bourgeois Pig, a dingy coffeehouse at the foot of Beachwood Canyon. He looked wary.

“Want a coffee?” asked Josh, Mr. Weiss’ assistant—or “entourager,” as his boss jokingly introduced him.

“No thanks, man,” Mr. Weiss said in a Long Island accent unmellowed by almost 20 years in Los Angeles. “That’ll get me too amped up. Who knows what’ll come outta my mouth.” He was wearing black sunglasses and a red U.S. Marine Corps baseball cap turned backward, and fingering an unlit Romeo y Julieta cigar.

Mr. Weiss’ nervousness with the press dates back to his days as the badly behaved writer-director of Amongst Friends, a film about young, affluent, aspiring gangsters (“Goodfellas meets Metropolitan”) that was the darling of the 1993 Sundance Film Festival. The film put Mr. Weiss, then 26, in the full glare of the media spotlight (he was photographed by Annie Leibovitz and Bruce Weber), but his self-important, swaggering tough-guy persona didn’t always come across well in print.

In one profile published in now-defunct Premiere magazine, Mr. Weiss, a former club promoter and dropout of the New School’s film program, coyly suggested—or at least did not deny—that he may have killed someone. He told The Observer he was misquoted, though, he allowed, “I’ve had some interaction with some unsavory characters in my life.”

Mr. Weiss does not deny that he has—or had—a temper; he once shut down the set of Amongst Friends over a lost cellphone and admits to “massive, screaming” fights with Mira Sorvino, who starred in the film. Now such youthful antics are being immortalized on the small screen via the fictional “suit”-hating auteur of Queens Boulevard and Medellin.

“We came up with the idea of putting a director in, and I wanted it to be Rob Weiss,” said Doug Ellin, the show’s creator, who went to high school with Mr. Weiss in the Five Towns section of Long Island and described him as “an extremely funny, slightly crazy, good-looking nutjob.”

Mr. Ellin even asked Mr. Weiss if he’d play “Walsh,” but Mr. Weiss declined. Instead, the role went to the Vincent Gallo-esque Rhys Coiro, to whom Mr. Weiss bears little outward resemblance. “I’m more Johnny Drama,” Mr. Weiss said, referring to Kevin Dillon’s physically well-maintained character. “I’m into grooming and metrosexual kind of shit. I’m the guy with like 900 products.”

In recent months, Billy Walsh has emerged as a surprise standout among Entourage’s ensemble of scenery-chompers, so much so that while the plan was to write him into five or six episodes this season, he will end up in nine or 10. According to Mr. Ellin, the character was becoming so dominant that he has received notes from HBO executives, warning him to “be mindful” about not overshadowing the show’s core cast: the buddy-family quartet of Vince, Drama, Turtle and “E,” and agent d’horreur, Ari Gold.

Mr. Ellin stressed that his own experience as a novice director (credits include Phat Beach) also informed the Walsh character, along with reading about Directors Gone Wild in books such as Final Cut and The Devil’s Candy. “I’ve taken all the crazy stories I’ve ever heard about directors,” he said, sitting in his Beverly Hills office, his blue and gold LeBron James Nike sneakers (“I only wear Nikes”) propped up on a coffee table. The Entourage episode in which a documentary filmmaker visits the haywire set of Medellin in South America was “an homage to Hearts of Darkness”—the documentary chronicling Francis Ford Coppola’s breakdown while making Apocalypse Now.

Nonetheless, it is Mr. Weiss who lies at the heart of Billy Walsh. And it’s not the first time filmic homage has been paid. He appeared as himself in Barry Levinson’s movie Jimmy Hollywood and surfaces, less flatteringly, in John Pierson’s indie-world tell-all Spike, Mike, Slackers and Dykes as, among other things, a “posturing director” and “Vanilla Weiss.” (Mr. Weiss had a falling out with Mr. Pierson in the aftermath of Amongst Friends, for which Mr. Pierson provided financial backing. “I have bad feelings about that guy,” he said of Mr. Pierson.)

And what do people have to say about Rob Weiss?

Actor Louis Lombardi (24, The Sopranos), who got his first acting break in Amongst Friends (his wardrobe of XXL track suits was provided courtesy of Mr. Weiss’ dad’s closet) recounted the grandiose, hyperbolic—indeed, Billy Walshian—way Mr. Weiss would carry on while making Amongst Friends, which had a cast of unknowns and was shot on a budget of just under $1 million (part of which was raised through gambling buddies of Mr. Weiss’ dad, who ran a casino junket business).

“His famous line was: ‘It’s gonna be epic!’” Mr. Lombardi said on the phone. “‘It’s gonna be huge, it’s gonna be unbelievable! You don’t know how fucking big this is gonna be! We’re gonna go to Sundance, you know how fucking big Sundance is, Lombardi?’

“He was a passionate person speaking in a crazy, exaggerated way,” Mr. Lombardi continued. “That’s why we loved Rob. He’s psychotic! He’s totally Billy Walsh.”

Amongst Friends never made much money—Mr. Weiss estimated he’s made about $35,000 from the film—but led to a three-picture deal at Universal; a bungalow adjacent to Ron Howard and Brian Grazer; Vogue photo shoots; “high-profile girls” such as Shannen Doherty, to whom Mr. Weiss was briefly engaged; and offers to direct films such as Good Will Hunting and American Psycho, which Mr. Weiss turned down because he wasn’t interested in projects that “didn’t come out of my head.”

Mr. Lombardi and other members of the L.I. crew followed Mr. Weiss out to L.A., moving into his West Hollywood apartment and assuming entourage-ish jobs, such as shuttling Mr. Weiss around to meetings with agents and producers. “Turtle is a little bit like me,” Mr. Lombardi said. “I would pick Rob up and drive him to meetings with, like, Oliver Stone. I’d be sitting in the meeting with him and Rob, and Oliver would say, ‘Go through my library of scripts! Any movie you want to make, I’ll make!’ Rob was like, ‘I don’t know what I want to do, I want to make a big movie.’ … He had a big ego, and he’d let people know.” Still, he said, “he is a loyal guy. He’s a good friend to have.”

Indeed, Mr. Lombardi and the actor Frank Medrano, another Amongst Friends alum, would often help themselves to Mr. Weiss’ expense account at Universal and run up $300 lunches at the studio commissary.

The gravy train was short-lived, however. Although Mr. Weiss wrote another script—Milk Bar, based on his experience as a club promoter—he never again directed a film, a fact he blames largely on his then-belief that nothing was good enough for his outsized talents.

“There are a million things from my past that were great opportunities and I didn’t step up,” Mr. Weiss said regretfully.

After spending his late 20’s living off rewrite jobs and TV scripts, Mr. Weiss found himself “fucking dead” at 30, at which point he made an effort to turn his life around (and mellow out). He started by enrolling in therapy.

“There were long stretches of sadness,” Mr. Weiss said. “You know, a lot of internal work.”

Professional salvation came in in late 2003, when he received a call from Mr. Ellin asking him to work on a pilot the latter man had written that had just been picked up by HBO. Mr. Weiss, still hanging on to his directing dream, at first said no, but capitulated when his agent reamed him out.

Now, he has his own production deal at the cable network (Milk Bar may yet see the light of day)—not to mention some perspective. “At the end of the day, I’m just a middle-class Jewish guy who writes for a living,” he said, and lit his cigar. “I mean, I’m a 40-year-old man now. I go ballistic now in my life probably because my blood sugar level’s low from not eating lunch. Billy Walsh is supposed to be the retro Rob Weiss, the guy who couldn’t be controlled, the rebel, the guy acting out of insecurity, fear—neurotic, passionate, volatile. I’m not that guy anymore.”