But it was the man who made them the money, bombastic Piven (typical quote: "When I'm done with you, it will make the battle of Normandy look like a game of f---ing paintball"), who rapidly became the star.
Since Entourage ended, Piven has moved on to the arguably less glamorous, but almost as high-profile, world of British costume drama with the eponymous role in ITV's Mr Selfridge, screened in more than 150 countries.
Critics have been more divided about Piven's performance as the visionary, philandering American who founded Britain's first department store, with The Hollywood Reporter saying he "seems out of his element paired with British actors who have this costume-drama thing in their DNA".
Others said they could see little difference between the two flamboyant dealmakers, Harry (Selfridge) and Ari.
"Harry is the antithesis of Ari," Piven retorts. "How many times has Harry raised his voice in three series? He doesn't have a hair-trigger temper, he doesn't lead through intimidation, he inspires his colleagues to be successful, whereas Ari leads with his ego. But Ari is certainly a lot of fun to play; he's such a drama queen."
Waiting for Piven earlier in the day, in a glamorous hotel bar in central London, I had been curious as to whether a similar drama queen was about to appear before me. In an era of carefully managed screen careers, there is something almost charmingly retro about how Piven seems to upset people.
The New York Post once asked rhetorically if he was "the biggest jerk in showbiz", after he had a hissy fit on a photo shoot. Then there was the 2009 Speed-the-Plow incident, more of which later, when Piven dropped out of a Broadway run of David Mamet's play two months before it was due to end, blaming high blood levels of mercury, from excessive fish consumption.
"My understanding is that he is leaving showbusiness to pursue a career as a thermometer," said Mamet.
Arbitration supported the story, but Piven was still mercilessly lampooned. The day after he quit, one of the play's co-stars, Raul Esparza, emotionally informed the audience in a curtain call: "Today was the first time I really enjoyed playing this show. I hope you weren't expecting a big TV star."
But when Piven arrives, chauffeured directly from the set of the fourth and final series of Mr Selfridge, he is less diva, more old-school thespian.
Stocky and hirsute, he is trying to be incognito in casual clothes, rather than the natty suits he normally favours, but the fact we're in a dark bar and he is in dark glasses with a baseball cap drawn low, only draws attention to him.
"Ah, I don't want to complain to you about being tired," he says with a sigh, perusing the drinks menu (the tequila that has been waiting for him is not to his liking, so he chooses another brand), before muttering, in reference to who knows what: "so surreal". There's an almost palpable intensity to him as he continually closes his eyes, while his pauses before responding to the simplest question feel longer than an Entourage episode.
There is also a fair amount of "luvvie" gushing - "So sweet of you to ask," he coos in response to an awkward question - insecurity and focus on craft.
But then Piven has greasepaint in his veins. The son of two actors from Evanston, north of Chicago, he attended their theatre school, whose other alumni include Rosanna Arquette and Cusack. ("We've known each other since we were eight," he says in reply to a question about whether they fell out after Cusack was ungracious about his old flatmate's late success.)
In his teens he came to London on a National Theatre exchange programme.
"I saw Judi Dench and Sir Anthony Hopkins in Antony and Cleopatra: life-changing."
"I'm a stage actor from Chicago," he says, leaning his forehead into his hands. "I have nothing in common with the Hollywood sharks of this world, who are motivated by money."
I'm surprised to hear this, given shooting of the Entourage movie was held up over "greedy" (in the words of producer Mark Wahlberg) squabbling about the cast's pay. In the end Piven received a reported £3 million (NZD$6.8m), to the chagrin of the other cast members who were paid "only" £1.2 million (NZD$2.7m). Wahlberg subsequently called the cast and apologised. "[They] came back to the table and a week later it was fixed."
"Ari Gold would not want to be my agent, I don't agree with his ideology," Piven says, visibly more animated now the tequila has kicked in. "He would not want one of his actors disappearing to London for four years to make a British series, he would not want someone like me who's always looking for really cool projects and doesn't care what the pay is."
It's probably more accurate to say he doesn't care what the pay is as long as he earns enough doing less "cool" things such as Entourage. The film has performed disappointingly at the US box office.
Piven, however, appears oblivious to this. "Listen, we're supposed to entertain and I feel like it's an entertaining movie. The response has been really gratifying to me."
The film has also been slated for its misogyny. "It's obviously a male fantasy; it's written by men," says Piven, who has never been married. "But I think this movie will really appeal to women because it's ultimately about loyalty and how family and an addition to your family will trump any of your petty distractions."
Mr Selfridge, on the other hand, is something of a female fantasy about shopping, as well as a fabulous advert for the department store.
Piven is a well-connected chap. A keen drummer, he jams with Stewart Copeland from The Police, has attended LA Lakers games with David Beckham and is good friends with John Cleese.
Obviously, he is less friendly with David Mamet. Piven looks sad. "That [mercury poisoning incident] was very unfortunate because I'm a guy who grew up on the stage. But I knew I was in trouble and not well. I'm still intravenously chelating [removing] mercury out of my system, yet in the States this unfortunate thing was received in a different way."
How much fish do you need to eat for it to poison you, I ask, eyeing the prawns Piven has just - possibly rashly - ordered.
"For 20 years that's all I ate," he explains, bravely popping one in his mouth. "Little did I know I was poisoning myself. There were other things going on. Never taking breaks and going from one job to the next, all part of the actor's journey. It's your job to find balance and I didn't have much of that."
Balance may come when Mr Selfridge wraps forever in October. Piven seems unlikely to be invited back to Broadway any time soon, and he was "gutted" to have to turn down a part in Chiraq, Spike Lee's musical about Chicago, because of a schedule clash.
"Such is an actor's life," says the man who describes himself as a Jewish Buddhist. "One of the many beautiful things I've learnt from your wonderful British actors is they don't want to be stuck in one show for too long. That's where I want to be now at this time in my life, exploring my range. I just want to do great work and I don't care what the scale is."
He is right - such a comment would have Ari Gold punching the wall in fury.
