Marvel’s film adaptations of comic book action adventures are expanding into a cosmos of their own
"If I had a key on my computer that said US$1.5 billion [$1.9 billion], I would press it every time." With its predecessor, 2012's The Avengers, ranking as the third biggest movie of all time, Joss Whedon believes that helming a superhero extravaganza like Avengers: Age of Ultron comes with its own pressures.
Best known as the creator of cult favourite television show Buffy the Vampire Slayer, the 50-year-old has in recent years reinvented himself as the driving force behind Marvel Comics' highly successful cinematic division, helping to get the Agents of SHIELD TV show off the ground as well as taking charge of Earth's Mightiest Heroes themselves.
Now, he's bringing the old team back together and introducing some new members for the all-important second mission.
"I don't think about things like that," he says. "I just try to write it as well as I can."
The culmination of the so-called Phase Two of Marvel movies, Age of Ultron follows on directly from Iron Man 3, Thor: The Dark World and Captain America: The Winter Soldier, if not last year's massively popular cosmic epic Guardians of the Galaxy and Whedon admits that having some story foundations laid by previous instalments can prove advantageous.
"That kind of makes it easier and harder," he says. "Having some restrictions can sometimes be frustrating but it can also be very useful because it means that the page is not so blank.
"A lot of questions have already been answered, so you know going in what you're getting to work with."
Dating back to 1968's The Avengers #68, Whedon has always intended to pit the Avengers against the megalomaniacal artificial intelligence who, through motion capture, is brought to sinister life by James Spader.
"I said that we should have Ultron in the second movie before I'd decided to make the first movie," Whedon recalls. "He's big, he's powerful and he's made of metal, so he's strong enough to take these guys on. And he has been angry for so long that he's a little unhinged and, as a bad guy, I can write that!"
As well as Ultron, the Avengers also face off against the misguided twin threat of malevolent spell-caster, the Scarlet Witch and super-speedster Quicksilver (Aaron Taylor-Johnson).
"I'm just thrilled to be a part of this whole world as it's a franchise that I've been watching since the first Iron Man," says Elizabeth Olsen (Wanda Maximoff). "Marvel makes such great films, with balance and depth of humour and action. They're all humans who have their own weaknesses and strengths, especially in this film."
And although his disembodied voice has been heard in the Iron Man films, Age of Ultron marks the first time Paul Bettany has appeared on-screen in a Marvel movie, albeit in the head-to-toe purple-skinned costume of Ultron's android offspring, the Vision.
"It was lovely to finally be on set with a bunch of people I'd supposedly been working with for 10 years but I'd never met," he says.
"But it's like a double-edged sword. Initially, my job was to turn up at the end after each movie had been shot, as I had the superhero power of explaining anything that was still unclear. I could clarify things by just talking and then, after about two hours, I'd go home.
"I couldn't imagine it being any better but being a proper part of this film has been like a dream. Everyone was so welcoming. They're a bunch of really happy actors and that makes for a nice environment to work in."
Although Jeremy Renner's Hawkeye and Scarlett Johansson's Black Widow haven't yet been granted their own solo outing, Whedon has considerably enhanced their roles, providing intriguing back-stories for both characters and simultaneously making them pivotal to the unfolding plot.
"What's great about playing this character is that she's based on something really grounded and very real that I could hold on to," says Johansson.
"She has experienced a lot of trauma and has never really been able to make active choices for herself but now she's finally ready to do that. "Unfortunately, it turns out to be kind of bad timing!"
As well as forming a special bond with the Hulk, the Russian assassin becomes romantically involved with the Green Goliath's alter ego, Bruce Banner (Mark Ruffalo).
"She's at a place where she might even be able to have a relationship with somebody," says Johansson.
"It could happen that way but, unfortunately, she has this kind of greater calling that she answers to, and she chooses that, which is almost heroic in a big or small way.
"She's also kind of content in herself, which is interesting because she's a really slippery fish as a character, because that's her job."
Although Johansson insists she "would be happy to put on the cat-suit again", Age of Ultron represents Whedon's swansong. He is determined to take "a long rest", but doesn't completely rule out returning at some future juncture.
"I don't think I'll ever go that far away because I love this stuff so much," he says. "But right now I have no immediate plans."
DC Comics' owners, Warner Bros, will finally launch its own sequence of superhero epics, starting with next year's Batman v Superman: Dawn of Justice, and Whedon believes Marvel is in excellent hands.
Taking in Benedict Cumberbatch's Doctor Strange and the first female-led Marvel movie in the shape of Captain Marvel, the recently announced Phase Three will span an ambitious four years, before climaxing with the two-part Avengers: Infinity War.
"What I love about what [Marvel Studios' president] Kevin Fiege is doing is that he's approaching each movie as a completely new idea, as a movie of a particular genre that happens to have superheroes in it," says Whedon.
"He's not interested in creating a formula, he's interested in creating a universe. As long as somebody who really cares is at the head, trying to create new versions of superhero movies and not just falling into a pattern, he will be able to sustain it for a long time to come."