Joss Whedon on The Avengers

It’s the world premiere of The Avengers and its stars are out in force on the Hollywood red carpet. Standing alongside Robert Downey Jr, Scarlett Johansson, Samuel L Jackson and others, and looking strangely out of place, is the film’s writer/director Joss Whedon. Red-bearded and grinning gleefully, his cheeks hot with excitement, it’s as if your mate from the pub has somehow managed to elbow his way into the line-up.

Yet it’s hard to think of a more perfect candidate to captain Marvel Comics’ Avengers. As a kid, Whedon was obsessed with Monty Python and comic books. Now 47, his interests remain pretty much the same. As the creator of the TV shows Buffy, Angel, Dollhouse and Firefly and the author of several Marvel comics, he is one of the sci-fi/comic/fantasy geeks that have inherited if not the earth, then definitely Hollywood.

We’ve had to wait a while for a film (his only other directorial effort was Serenity back in 2005), but now two have come along at once: The Cabin in the Woods, which Whedon co-wrote with his protegé Drew Goddard, is out in July, and right now it's possibly the biggest superhero movie ever. "I have absolutely always wanted to make a superhero movie," Whedon says happily. "It’s a dream come true." He pauses, grinning. "But you do have to be careful what you wish for. I wished for superheroes and they gave me six in one movie!"

He’s not kidding. The Avengers is a superhero supergroup extravaganza featuring the Hulk (Mark Ruffalo), Iron Man (Robert Downey Jr), Thor (Chris Hemsworth) and Captain America (Chris Evans), as well as Hawkeye (Jeremy Renner) and Black Widow (Scarlett Johansson). Gathered together by Nick Fury (Samuel L Jackson), the director of international peacekeeping agency SHIELD, they must band together to – you guessed it – save the world. The film is the culmination of Marvel films that began with Iron Man in 2008, placing all the eggs that are sacred to comic aficionados into one very big basket.

For a man about to have judgement passed on his work by the world’s most diehard fans, Whedon is calm: "There wasn’t any pressure from fans," he says matter-of-factly, "because I’m a fan myself from way back. It wasn’t like…" – he puts on snooty accent and raises his nose in the air – "Oh, how can I please those people? Those people who read the funny books?" He grins. "No. This is me. Target audience? Me."

Nothing seems to daunt Whedon. The writer-director-producer has made a career of humanising his superheroes – think of vulnerable and funny Buffy, or the alienated outcasts in Serenity. This time Whedon’s heroes display their flaws to the full. Iron Man is arrogant, Captain America is out of touch and the Hulk has some serious anger issues. Whedon has a knack for characters that aren’t perfect, but that’s what makes them real.

In person and close up, he looks eternally young, with the sweet, innocent face of a 1950s child actor – flushed and eager to please. He sits in a Beverly Hills hotel suite holding court to a non-stop stream of journalists with good-natured humour and what looks like total ease. Born in New York, he is a third-generation TV writer: both his father and grandfather were noted small screen scribes. His dad worked on The Golden Girls and Whedon got his start on Roseanne. "It was always around me and seemed a perfectly sensible thing to pursue," he says. "My dad would go to work every day and write in a room full of funny people. He enjoyed it. I know great writers who find the process agonising but to me, writing has always been sheer joy."

With The Avengers, he adds, it was vital to make it appeal across the board: "You have to make sure that anyone who doesn’t give a cuss about the Marvel universe is still having a wonderful time." And we do. The film is full of the usual Hollywood hoopla: big explosions, complicated costumes and sly asides, but without the bloat. Whedon has made a massive movie feel smaller, more intimate and somehow even realistic – despite the lumbering green Hulk and a flying fish thing from outer space. "I always want to make sure I’m telling a story about people that I care about," he says. So characters bicker, they struggle with their emotions, they actually talk to each other.

"The movie had been cast with powerhouse actors," Whedon explains of taking on the script. "So you don’t want to invite them to the party and have nothing for them to do. The biggest inspiration for me writing this was The Dirty Dozen, because that’s a film about people who don’t like each other and don’t belong together. And the structure of that movie is: get to know these people, watch them interact and then, late in the game, let them kill the bad guys.

"People think it’s all about the mission, but it’s all about the team, really. When you do a film like this you have to make sure each character matters as much as the others, but in a different way. That was something that was really important to me. Making sure everyone mattered." And here, perhaps, is the secret to Whedon’s longstanding success. By making sure his characters matter, he makes the audience feel as if we matter too.

"I don’t like uncertainty. I don’t play poker. I don’t like bluffing," Whedon says later at a press conference, looking as happy as a clam amid microphones and camera flashes. "But what I do like is hiking. And that’s what filmmaking is. It’s a hike. It’s challenging and exhausting and you don’t know what the terrain is going to be or necessarily even which direction you’re going in… but it sure is beautiful."

The Avengers screens from Wed 25 April