Here's how a letter from 'Aquaman' 'director' James Cameron changed the course of 'Entourage'

James Cameron doesn't make flops... even fake ones. Fifteen years ago, the Titanic director changed the course of a major Entourage storyline when he wrote a letter to the creator, Doug Ellin, expressing his unhappiness with the HBO show's direction. Specifically Cameron was unhappy that Aquaman — his fictional blockbuster about the very real DC Comics superhero — was revealed to be a titanic box-office failure in the opening episodes of Season 3, which premiered on June 11, 2006. "The plan was for Aquaman to be a bomb," Ellin tells Yahoo Entertainment. "He wrote me a letter of why it had to be successful." (Watch our video interview above.)

The Aquaman saga, and Cameron's role in it, remains one of Entourage's best-remembered storylines. Ellin originally conceived of the idea in the show's second season, which found fictional movie star, Vincent Chase (Adrien Grenier), being offered the chance to headline a comic book blockbuster directed by one of the most successful filmmakers in Hollywood history. Cameron agreed to play himself on the series, and appeared on multiple episodes as Aquaman went into production. "In 2004, Aquaman sounded absolutely ridiculous to me," Ellin says now. "The only director on Earth at that moment that I thought made it sound good was James Cameron."

Although Cameron has a reputation as a demanding director, Ellin says he was an easy actor work with. The writer says he also got a sneak preview of the filmmaker's creative process. "I swear to you this is true: Mr. Cameron comes out of his trailer, and says, 'Doug, how do you write these guys? They sound so real. I'm writing these blue people.' I didn't even know what he was writing!" Those "blue people," of course, were eventually revealed to be the main characters in Cameron's 2009 global blockbuster Avatar, which recently reclaimed its status as the highest-grossing movie of all time.

When Season 2 wrapped, Cameron had no inkling where Ellin intended to take the story. But during the year-long hiatus between seasons, he heard from the show's post-production supervisor Janice Tashjian that Chase's superhero dreams would be over before they began. At that point, Cameron sat down and penned his letter to Ellin demanding a rewrite. "And he was right," Ellen says now. "Like, how dare I do this to him when he was kind enough to come in and do this favor for us?"

"What I didn't realize is that a lot of the real guests who came on the show felt that people really looked at the show as real," Ellin continues, adding that he still has Cameron's note all these years later. "You can't just disparage [them] in some way. It's a fake TV show, but that was kind of the thing that would blur the lines. So he wrote me saying that this movie has to be successful, not in a nasty way at all, and it changed the storyline."

As it turned out, flipping Aquaman from a flop to a hit opened the door for Ellin to oversee one of his favorite episodes of the series: Season 3's sophomore installment, "One Day in the Valley." It's an episode he's talked extensively about on Victory: The Podcast, the Entourage podcast he hosts with fellow show veterans Kevin Connolly and Kevin Dillon. Set on Aquaman's opening day, the plot involves Vince and his usual crew — "E"(Kevin Connolly), "Drama" (Kevin Dillon) and "Turtle" (Jerry Ferrara) — stranded in the San Fernando Valley as rolling blackouts threaten to beach the movie's chances at success.

But when the numbers come rolling in, Cameron's film ends up surpassing what was then the current record-holder for biggest opening-weekend, Tobey Maguire's Spider-Man. (Funnily enough, Cameron nearly directed a film version of Marvel's beloved wall-crawler in the 1990s, but moved on before the character's tangled legal history was resolved.)

According to Ellin, the "Aquaman beats Spider-Man" storyline was equally inspired by Cameron's letter and Maguire's own experience headlining a major comic book hit. "Kevin Connolly's very close with Tobey," Ellin reveals. "I remember him telling me that he was with Tobey when they got the call that Spider-Man broke the record. Talk about wish-fulfillment! So that's where that storyline came from."

And Ellin continued to mine Maguire's life for material as Season 3 continued. After Aquaman becomes a certified hit, Vince waffles on returning for the sequel leading the studio to threaten to replace him. That storyline mirrored what happened on Spider-Man 2, when Sony Pictures contemplated recasting the role with Jake Gyllenhaal after Maguire asked for more money and disclosed a potential production-delaying back injury. "I've never asked Tobey what was going on behind the scenes," Ellin says. "I took what I read and ingested it, and maybe asked Kevin Connolly a couple of questions. But yes, that was inspired by that [story]. How close it was to reality, I have no idea."

A decade after Entourage's fake Aquaman movie (and the CW's abandoned TV pilot), the aquatic hero came to the big screen for real in Batman v. Superman: Dawn of Justice. Zack Snyder's 2016 film introduced Jason Momoa as Aquaman, a role he reprised in 2017's Justice League and his own 2018 solo adventure, directed by James Wan. In a case of life imitating art, Wan's Aquaman became a Cameron-sized hit, grossing more than $1 billion worldwide. Speaking with Yahoo Entertainment in 2019, Cameron called the movie "great fun," while also pointing out how it would have differed from his vision.

"I never could have made that film, because it requires this kind of total dreamlike disconnection from any sense of physics or reality," Cameron said. "People just kind of zoom around underwater, because they propel themselves mentally, I guess, I don’t know. But it’s cool! You buy it on its own terms." He also took a sly dig at the movie's underwhelming — in relation to Avatar, anyway — billion-dollar gross. "Come back when you hit your second billion and you’re on your way to your third — then we’ll talk."

Ellin also says that he enjoyed Wan's film, but adds that if he could only see one Aquaman movie it would be Cameron's. "When James Cameron makes a movie, there's nothing like it. He's an off-the-charts genius. I appreciate the movie they did, but if James Cameron did it, it would be a whole other animal."

Entourage is currently streaming on HBO Max.

Watch our full conversation with Doug Ellin on YouTube

Read more from Yahoo Entertainment:

• 'Entourage' creator Doug Ellin on how 'wave of righteous PC culture' impacted HBO show's legacy

• 'Aquaman' 'director' James Cameron tells us how he really feels about James Wan's 'Aquaman'

• Jason Momoa on being a biracial 'Aquaman': 'It's an honor to be a brown-skinned superhero'