The morning Joss Whedon picks up the phone to talk about Avengers: Age Of Ultron, he sounds as if the two years he’s devoted to writing and directing the hotly anticipated superhero sequel have left him a husk of a man, unable to discern real life from comic-book lore. “Reality, what’s that?” he mumbles. “Is that a thing? Is that what the kids are doing now? I work on the movie, go to sleep, work on the movie, go to sleep. When will I be ready to stop? About three months ago.”

He pauses, the fog surrounding him fades, and he’s able to summon up a little indignation in response to the observation that he might not initially have seemed the ideal choice to helm a chunk of the Marvel Cinematic Universe as vast and over-populated as The Avengers franchise. “You could choose to look at it as: ‘Why did they give this guy a break?’ But I feel like I’ve spent my entire career being bred for this job. I directed TV shows and a movie about superhero teams. I’ve written Marvel comics. It’s not such a surprise to me as it seems to be to everybody else.”

The film team review The Avengers: Age of Ultron Guardian

My intention was not to cast aspersions at Whedon’s competence. By his own admission, he doesn’t work well under the thumb of insensitive overseers. His first television break was working on Roseanne Barr’s show when she was at her most tyrannical. As a screenwriter-for-hire, his stints working on movies from Toy Story to Alien: Resurrection to X-Men proved frustrating.

Whedon’s authorial voice only bloomed when he was given complete autonomy over his TV creations, most famously Buffy The Vampire Slayer. How easy was it for someone so in control of his own worldview to transition into serving the Marvel vision? “You’re subservient to the process and it can be very gruelling, but we go in with an understanding that this is going to be a ‘Joss Whedon Marvel film’. It’s going to reek of my sweat and my blood by the time it’s done, so it doesn’t feel like it’s in service of anything except the narrative, which is my own creation.”

Few could deny that, with The Avengers franchise, Whedon has pulled off an amazing feat. Bringing together all those heroes, giving them an equal playing field (OK, maybe Hawkeye picked the short straw) and finally getting The Hulk right after the two previous movie attempts had failed, was an extraordinary act of multi-million-pound plate spinning. The reason DC is rushing to get a Justice League movie off the ground, the reason movie studios are falling all over themselves to build their own giant interlocking movie universes featuring combinations of beloved characters, is all down to the blueprint Joss Whedon laid down with The Avengers.

For this second instalment, the all-star lineup of Iron Man, The Hulk, Black Widow, Captain America, Hawkeye and Thor don’t just have to contend with Ultron, Tony Stark’s peacekeeping device-turned-rampaging, endlessly replicating doomsday machine (“That is one angry robot,” chuckles Whedon) but evil twins Quicksilver (Aaron Taylor-Johnson) and Scarlet Witch (Elizabeth Olsen). That seems like a lot of superheroes for one gargantuan summer blockbuster.

Joss do it: Whedon directs Jeremy Renner, who plays Hawkeye. Photograph: Jay Maidment/PR

“It is tough,” he agrees. “There’s basically 11 main characters in this movie, which is quite frankly too much. I may die. Thor’s probably the hardest to write at any point because his adventures tend towards the legendary and you can only have so much of that before people start to go: ‘Wait a minute, whaaaat?’ But the fun of the thing is taking Hawkeye and the people who don’t have their own franchises – Banner, Natasha, the new guys – and delving into their lives.”

I don’t really think about the fans. I think about what I want Joss Whedon

I offer the opinion that Marvel’s current wave of success comes from the fact that it concentrates on satisfying the segment of the audience previously ignored by superhero movies: the loyal, hardcore comic-book fans. Whedon could not disagree more. “They are the last people you need to worry about. I don’t really think about the fans. I think about what I want because I’m the guy who’s taking the comic book he read as a kid and turning it into a movie that I want to see as much as I want people to see it. [Marvel Studios president] Kevin Feige and I are fanboys, straight up. But we also know that if you just spend your time catering to the fans you make something that is hermetically sealed. The first question we always ask is: ‘What is the way in for someone who has never seen a superhero movie?’ You need to be thinking about everybody all the time.”

Directing Elizabeth Olsen and Jeremy Renner. Photograph: Allstar/MARVEL STUDIOS/Sportsphoto Ltd./Allstar

An Avengers sequel seems like a pretty safe box-office bet – movie trade bible Variety is currently predicting a record-busting $200m opening weekend – but is there a hidden fear that the movie that ends the superhero hot streak that began with Iron Man in 2008 could be right around the corner? “Oh yes,” Whedon perks up, clearly more comfortable discussing failure than success. “That’s constant for me as an artist and it’s constant for Marvel as a studio, especially because Guardians [Of The Galaxy] was such a risk and it did so well. It’s like: when are we going to be punished? Nobody plays the stock market thinking it’s always going to be booming. You’re always waiting for people to call bullshit on things you do. I’m really hoping that doesn’t happen until I’ve packed up my office, and Avengers 2 doesn’t mark the start of the great decline.”

I believe that the non-franchise films still have a place in this world Joss Whedon

Age Of Ultron marks Whedon’s swan song in the Avengers director chair but it’s hardly the end of the saga. The two-part Infinity War story has been set in stone for 2018 and 2019, along with eight other Marvel titles. DC, looking to get its comic-book universe up and running, has also announced a hefty release schedule. In tandem with a fresh batch of X-Men sequels, Spider-man spin-offs and Fantastic Four reboots, that’s at least 25 new superhero movies over the next five years.

I tell Whedon that people look at that schedule and, with heads in hands, declare it the death of movies. “Honestly,” he says, “it’s going to be difficult to maintain quality but I don’t think that schedule equals the death of movies. I think it’s a new paradigm. To me, the Reagan era seemed to have heralded the death of movies. It was the end of the great, insane, introspective serious 70s movies. But I’m sure you can point to any time since the birth of cinema as the death of cinema. The question for me is only ever: are we making good movies? Now you don’t always hit and it’s frustrating as hell that it feels to a lot of people that this is all that’s going on but there are a lot of other movies being made and I believe that the non-franchise films still have a place in this world and, God willing, maybe I’ll go on to do something that isn’t part of seven other movies.”

Fanger management: Sarah Michelle Gellar as Buffy The Vampire Slayer. Photograph: Everett/REX Shutterstock

As tirelessly as Whedon has toiled on his Marvel movies, his TV creations and the emotional hell he puts them through is where his real strength lies. Even undemanding TV audiences rolled their eyes when Buffy The Vampire Slayer made its debut in 1997. A further iteration of a failed, strained, camp, teen-pandering horror comedy from 1992? Except the TV adaptation of Buffy was the exact opposite. Buffy’s rich subtext, its humour, its feminism and willingness to venture deep into the dark places from which teen shows had previously shied won it critical adoration and an audience whose devotion belied its meagre ratings. It also put Joss Whedon in the strange position of being a creator whose name was enough to get a show on TV but not enough to keep it there. Buffy’s spin-off series, Angel, took a few series to find its feet and was cancelled as its audience was growing. The much-loved space opera Firefly received zero promotion, had its episodes aired out of sequence and was cancelled after 11 airings. While Whedon has immersed himself on these colossal movies, TV has changed. There are so many more outlets hungry for fresh content. “What?” he exclaims, affecting a shocked tone. “You mean there’s cable and, wait a minute, boobies? No one told me. How long has this been going on? Oh, a decade? Oh, I’m a little dim.”

With Scarlett Johansson. Photograph: MARVEL STUDIOS/Allstar/MARVEL STUDIOS

Whedon’s small-screen universe may have been confined to network TV with its attendant limitations, but his world was always filled with flawed, dark and damaged characters. In contrast, the big-screen superhero world is populated by special, larger-than-life figures who always do right. “That’s the most interesting and potentially dangerous thing about making superhero movies. They’re all bigger than life and when part of you wants to say, we have to make life better for normal people who are actual size, you have to tread lightly. You don’t want to turn into Leni Riefenstahl.”

The X-Files is back. Twin Peaks is (maybe) back. Lots of other cult shows that couldn’t find audiences are coming back. I point out to Whedon that he is the creator of several series viewers might like to see make similar comebacks. “I love these shows,” he says. “And I think about it sometimes but I’m very wary of the monkey’s paw where something you love comes back but not quite as good. Firefly was filmed with a sword of Damocles over its head every episode. That meant every episode had to be as good as we could humanly make it. We didn’t have time to experiment or explore ideas that were just OK. We had to know what the next story needed to be. It drove us in a way we might not have been driven if we were in a place that had supported what we were doing. I think about revisiting a lot of those things but I also think, well, what else you got? I don’t want to be done creating new universes.”

Reality may have to wait a while longer.

The Avengers: Age Of Ultron is out in cinemas on 23 April