On set with the Avengers

The first Avengers is the most successful film of all time not to have been directed by James Cameron

BBC Entertainment correspondent Lizo Mzimba goes behind the scenes on Marvel's latest blockbuster, Avengers: Age of Ultron.

It's a sunny Friday in August 2014. All around Shepperton Studios, threatening signs warn against unauthorised entry to the soundstages.

The film they're keeping under wraps is of course Avengers: Age of Ultron, although the notices only refer to the movie's code name "Afterparty".

And what a party the original Avengers turned out to be. It took more than $1.5 billion (£991 million)at the global box office, and delighted millions of fans across the world with its mix of irreverent humour and budget-busting action sequences.

I'm here on what is day 89 of a 93 day shoot. During a brief break in filming, the movie's writer and director Joss Whedon confesses to feeling the pressure - but it's not the pressure of outside expectation. It's the pressure he puts on himself.

"The fact is there's a certain amount of expectation, obviously," he admits. "But for me the expectation is, can I make a better movie? Can I make this more interesting? Can I push myself as a filmmaker, can I push the actors? Can I expand the Marvel universe in the way that it should be expanded? Not just make it bigger, but make it deeper."

As writer and director, Whedon had to juggle up to 10 characters at a time while maintaining the narrative thrust of the story

Last week, a few days before the film's release, Whedon is still tired. Even more than the last time we met.

"It feels like I'm at the end of a two or three year journey," he sighs, "and it was all uphill."

"When you start out, it's all perfect in your head. When you work with the actors, it gets better.

"And then at some point you've been editing it for so long you start thinking 'What am I? What's happening?' and you forget why you ever showed up.

"And you despair. It's a very bleak experience."

Luckily, he's now had the privilege of watching audiences' enjoyment of the finished film - and it's been something of a vindication.

"When you get a reaction, particularly if it's something you fought for. Then it's pretty wonderful."

It's hard to overstate how important Joss Whedon has become to the series of films that make up the Marvel Cinematic Universe. He's been a creative consultant on the franchise for three years, in addition to his duties on the Avengers sequel.

Chris Hemsworth, who plays Thor, is full of praise for the "fantastic writer and director". Speaking after a long night shoot, Hemsworth says Whedon has done an exceptional job keeping one of cinema's biggest-ever productions on track.

"I'm as impressed, if not more so, than I was on the first one working with him," he says. "There's got to be a sense of confidence with that success, but he has just such a great self-deprecating sort of humour, and I'm sure he'd tell you the opposite. Look what we're involved with, so he's done something right."

Newcomer Elizabeth Olsen, who plays Scarlet Witch, might be Thor's on-screen opponent, but she shares his appreciation for the director.

"The guys from the first Avengers all make jokes saying 'In Joss we trust' because, even though you maybe feel silly some days at work, Joss says, 'no, it looked awesome'.

"Then you go to [fan convention] ComicCon and you see the teaser they put together and you're like 'In Joss we trust!' You feel really taken care of. More so than I have on any other film."

The cast gather to promote the movie at 2014's ComicCon

Olsen's character is a welcome female addition to the male-dominated series. "I know that Joss doesn't want it to be a boys' club. He wants it to be what the comics are, which is a balance of men and women," she emphasises.

"I think it's fun for him to have been able to develop the character together, as opposed to the way he inherits roles from all these other franchises."

Whedon, who in the past tried to bring Wonderwoman to the big screen, agrees that there is some way to go in balancing genders in the superhero genre.

"There is a genuine sort of recalcitrant, intractable sexism and old fashioned sort of quiet misogyny that goes on. Now I'm not saying it's in Marvel more than other places, 'cos there's no one out there who's really doing it in this genre.

"I think at some point if Marvel doesn't step up, someone's going to sweep that away from them. I mean Hunger Games already has," he says.

"Marvel is in the position of making a statement simply by making one of those movies, which I think would be a good thing to do. But it has to be a good movie and it has to be a good character."

And on that note Whedon returns to the adjoining Stage J. Among the scenes he's overseeing today is one of the film's less complex moments.

The Hulk's alter ego Bruce Banner is emerging from the Avengers jet known as the Quinjet. He looks around warily calling out "Is this a code green, guys?"

Between every take the director and Mark Ruffalo who plays Banner/ The Hulk confer, eager to get the scene just right, surrounded by dozens of studio crew.

Ruffalo plays both sides of the Hulk's Jekyll and Hyde personality

Once he's wrapped the sequence a few hours later, Ruffalo says he's taken the view that he's not going to worry too much about the Hulk-sized pressure the film is facing. Instead, he's simply enjoying the experience.

"It's actually been quite a bit of fun. It went well the first time so we're not battling against people's negative expectations, or lack of expectation, or whatever hurdles that I had to personally overcome.

"It's just a great group who, after the success of the first one, have become very close and it's a nice little family. And it's good to be back."

Avengers: Age of Ultron opened in UK cinemas this week.