With enormous, CGI-filled tentpoles like "The Avengers," it's easy to forget that there's a huge team of people whose job it was to make the movie and that behind all of them, there's one guy making sure it all doesn't go to hell. That, as MTV News learned on the set of "Avengers: Age of Ultron," can be a very exhausting job.

Even as he cracked jokes with the group of journalists visiting Shepperton Studios, director Joss Whedon appeared to be a slightly more rundown version of his ever-witty self. It wasn't that his eyes had enormous bags under them or that he looked as if he hadn't slept in days, which we guess is possible. Instead, as he sat on a folding chair in the center of a messy lab in Avengers Tower, Whedon had the air of someone spinning more multi-million dollar plates than most mortals could handle.

But according to him, that was all part of the plan.

Whedon has never kept it a secret that "The Avengers," the $1.5 billion worldwide phenomenon and the director's second feature, left him almost completely drained. His stated purpose for filming the hush-hush, ultra-low-budget version of "Much Ado About Nothing" was to revitalize himself creatively between pre- and post-production of the superhero team-up movie. And yet here he was doing the whole thing over again.

This time, however, Whedon is one Avengers movie wiser, and his approach to "Age of Ultron" was to give himself entirely to the massive task at hand, "like a Christian to a lion."

"I was a raggedy man when I made that film," Whedon told the assembled journalists. "It did take a lot out of me. Going in this time, I just had to sort of recalibrate me entire existence and throw myself into it more wholeheartedly and say, 'Okay, actually make it harder to make them last.' I’m gonna just invest myself in every part of it: in every production meeting, every location scout, and every question about a prop that I’d like to avoid. I might even work harder on the script."

Making a sequel to "The Avengers" wasn't something he allowed himself to consider during production on the first film, even if was more or less guaranteed to succeed. "I didn’t actually want to make the [sequel] necessarily," Whedon said. "I was ragged from the first one, and so I just turned off my brain. I was like, 'Do not think of cool ideas for the next one. Just get through this.' "

Once "The Avengers" was an undeniable smash, Disney came to Whedon with their offer for him to direct the sequel, which forced him to make up his mind. The test for whether he could find it in himself to make another superhero team movie was surprisingly simple. "I went to a pub and sat down with my notebook," Whedon said. "About 45 minutes later, my notebook was filled. I texted my agent, 'Yup, and I have so many things to say.' I was kind of surprised."

Now that Whedon was officially on-board for another round with Iron Man, Captain America, Black Widow, Hawkeye, Thor and the Hulk, he could act on one of his earliest ideas for the series.

Back before he had signed to direct "The Avengers," Whedon told Marvel Studios that if the movie were to succeed and merit a sequel, they should make Ultron the villain. Now, we're only months away from seeing that movie, which Whedon describes as very personal. The crux of his idea for "Age of Ultron" is that the hyper-intelligent robot believes he can see through the façade that the Avengers put up. As the villain explains in the trailer, "You want to protect the world, but you don't want it to change." Ultron's plan to protect and change the world means the destruction of the team.

"The important thing for me is that [Ultron] is not this external thing. He’s not, you know, 'Independence Day.' I’m not criticizing that movie, but I’m saying that it’s not like we spent some time on the alien going, 'Oh, I hate that Will Smith! Punched me right in the face! The first day there!' When [Ultron] is in his scenes, you want to feel like he will never understand that he’s not the hero."

To illustrate this, Whedon is painting on an international canvas that takes place in at least four different countries and involves a superhero roster of such enormity that one of the biggest challenges with the script was making sure all of it felt like one story. Not only do the Avengers have to contend with a science experiment gone wrong, but there's the matter of Jarvis-made-man The Vision (Paul Bettany) and two products of the "age of miracles," Scarlet Witch (Elizabeth Olsen) and Quicksilver (Aaron Taylor-Johnson).

And yet with all of the complexity involved in making one movie, "Avengers: Age of Ultron" can't be a singular story. It will exist as just one tale in an interweaving system of films that aims to tell one enormous narrative. The Marvel Cinematic Universe has been fueled by huge ambition on the part of Kevin Feige and the rest of the studio, but on that set in May, all Whedon could manage was to focus on the impossible task at hand.

"There comes a point in filming when you are writing, filming and editing, and you cannot make a grocery list. I haven’t had a good idea about anything. I’m so excited that I’m wearing underwear. That I got that right today," Whedon said. "We’re just past the halfway point, and I’m still finessing and finessing and finessing, and I got nothing. So it's I do this, I go home, I rewrite, I go to sleep. I do this, I go home, I rewrite, I go to sleep."

"Avengers: Age of Ultron" opens in theaters on May 1.