But when their first self-financed project, Iron Man, unexpectedly came within kissing distance of $100 million on its opening weekend, Feige announced the studio was immediately starting work on a sequel, as well as new films starring Captain America and Thor, two more then B-list figures from the publisher's roster, to go with an already-completed Incredible Hulk picture that would be released in two months' time.

"I remember being excited that people would catch on that we were going to interconnect them, and, like, nobody cared," says Feige, now 45 and dressed in regulation blockbuster producer garb: charcoal polo shirt, navy suit jacket, branded baseball cap. "The story was the success of Iron Man, and at the end, it was 'Oh, now they're going to try this.'"

Eleven years later, this has grown into the Marvel Cinematic Universe, a series of 21 interlocking blockbusters that didn't unfold in an orderly fashion like sequels, but came in from all angles like the original comics, each one bearing new stars, plots, surprises, even worlds. The formula has been such an industry-warping hit - the films even tend to go down well with critics, not least this one - that the MCU's global takings to date total $18.5 billion, roughly equal to the gross domestic product of the European Union.

Joe Russo, a Marvel Studios stalwart who with his brother Anthony has directed four of Marvel's films including its latest, describes it as "a grand mosaic - 11 franchises that have been interwoven into one big narrative." For Feige's first leading man, Robert Downey Jr, aka Iron Man, it has been "the coolest relay race in the history of entertainment".

But the 22nd film is about to do something new: wipe the slate. Following the catastrophic Thanos Snap that scrubbed out half of all life in the universe at the end of last year's The Avengers: Infinity War, Endgame's plot is such a closely guarded secret, even its cast claim not to know how it fits together yet. "I've worked in it long enough to know you just have to trust in the machine," says Paul Rudd, aka Ant-Man. One thing is certain, though. Some of Rudd's co-stars, and possibly all four core Avengers - Downey Jr's Iron Man, Chris Hemsworth's Thor, Chris Evans's Captain America and Mark Ruffalo's Incredible Hulk - won't be back for film 23.

Along with the usually interview-shy Feige, the cast have assembled at the InterContinental Hotel in downtown Los Angeles to bid farewell to a franchise that has spawned envious imitators at every other studio in town, with varying degrees of success. (Lucasfilm tried to apply it to Star Wars, with its Han Solo and Death Star plan-nabbing spin-offs.) The atmosphere in the room is strange: mid-conversation, I often feel as if I'm at an upbeat wake. As Ruffalo says, "We're all talking like we're dead."

On its release, Endgame is expected to amass at least $1 billion in box-office receipts within a couple of weeks: a far cry from the studio's modest 2008 targets, which Feige describes as roughly in line with earlier C-list Marvel projects such as Fantastic Four, Daredevil and Elektra.

It's worth remembering these characters seemed too outlandish for mainstream success even a decade ago: Hemsworth recalls privately wondering on the set of Thor "whether this thing was even going to make it into cinemas, past DVD". It was only when the first Avengers film passed $1 billion in 2012 - "the first of our films Disney released, and the biggest it had ever done until Star Wars: The Force Awakens," Feige notes, with a glint of pride - that the actors realised this nerdy reverie had cohered into a pop-culture megalith.

The groundwork had been done more than half a century ago by Marvel's own stable of comic-book writers and artists, who dreamt up these latter-day box office colossi at Marvel's Madison Avenue offices, known affectionately as the Bullpen. Foremost among them was Stan Lee, whose death last November at the age of 95 marks the end of an era every bit as much as the Avengers' final screen appearance.

Feige's last conversation with Lee came two weeks before his death: it "was about upcoming things, and his enthusiasm for what was next". They ran through the eight as-yet-unidentified films Feige plans to release under the Marvel banner between 2020 and 2022, "and what I wanted to talk about was the characters - what the intention was when they were created, and how we might be able to improve it." But Lee's mind was elsewhere. "All he wanted to talk about was his cameos," Feige laughs. Lee had been popping up, Alfred Hitchcock-style, in almost every Marvel-based film since 2000's X-Men, and he had already drawn up a list of guises for the forthcoming instalments.

"For me, the cameos had always just seemed like the cherry on top of this amazing career," says Feige. "But I've since realised that to him they stood for something more. I think it was that he was able to stand beside these characters he'd co-created on a stage that was much bigger than he ever anticipated. I mean, these movies are bigger than I ever anticipated. So what it was like for him, I can only imagine."

As for the cast, even they weren't convinced things were working until the first Avengers film went out into the world.

"Once the cross-pollination began, it was like 'Oh, this thing could go on'," says Don Cheadle, who joined the franchise in 2010 as Iron Man's trusty wingman, War Machine. Ruffalo nods in agreement. "That first Avengers film exceeded anything that I thought it was going to do. And ever since, I've always been like, 'When's the piano going to fall?'," he says, looking anxiously upwards. "So I think it's smart for them to have a finality; to end on a high note."

Recently, the franchise has been at the forefront of bringing an ever-more-diverse range of heroes to the screen, though it's debatable whether Marvel has been driving a broader cultural shift or just shrewdly keeping up with one. But the changing tastes of the past decade are mirrored in the franchise - not least in its attitude towards female leads.

In the unreconstructed days of 2010, Scarlett Johansson's Black Widow served a primarily decorative function: there were scenes in Iron Man 2 of her getting undressed in the back of a limousine, and Downey Jr's Tony Stark leafing approvingly through shots of her in lingerie. Johansson describes her as "a sort of glorified sexy secretary, who had all these skills on the side. But by Avengers 2, she had become part of the group dynamic."

Given that recent female additions, from Brie Larson's Captain Marvel to Danai Gurira's Okoye, were able to arrive on the scene more fully formed, does she regret that? "That's just how it had to play out at that time," Johansson shrugs.

"It was what could be accepted. Then the audience, the fans, who are obviously a huge part of this movement for diversity on screen, became more vocal - there were more ways for people to speak up about what they wanted to see, and they wanted to see their own lives reflected on screen, their own ideals and aspirations. At times it felt a little like we were playing a game of catch-up."

Disney's recent acquisition of Fox returned the latter studio's mini-Marvel portfolio to the mother ship, meaning Feige now has the Fantastic Four and the X-Men, including Deadpool, at his disposal. Any in-house reboots, however, are "a long way off": the next phase of releases are a mix of sequels, fresh characters from the Marvel vault, and perhaps "versions of some characters we've already introduced". Kung fu master Shang-Chi and the cosmos-trotting Eternals are thought to be among the new recruits.

Sorry, who? Eleven years ago, many would have asked the same question of Black Panther, Captain Marvel and the Guardians of the Galaxy, but that hasn't stopped them amassing a billion dollars plus apiece. If this universe has an edge, it hasn't yet come into sight. The Telegraph, UK