Joss Whedon says he wanted a challenge with Avengers: Age Of Ultron.
The filmmaker is back in the director's chair for the upcoming sequel, which sees Aaron Taylor-Johnson, Elizabeth Olsen and James Spader joining the cast.
"Some things are easier, there's definitely a shorthand with the actors and the studio. But I set out to make it harder," the director said.
"I wanted this movie to be harder to make than the last one and hopefully that will show up on screen so it will be more interesting," he continued, adding: "I can't wait to be done!"
Of the new cast members, Joss said: "It's so wonderful. They're old-time movie stars, consummate professionals, extraordinary actors and idiots - just big goofs. And that energy - they're still thrilled and uncertain. It's really fun."
The 50-year-old director, who is keeping the plot under wraps, teased that the sequel would include "just a lot of talk, people sitting around talking about their feelings. It's going to be very, very tense."
Chris Hemsworth, Robert Downey Jr, Scarlett Johansson, Chris Evans, Mark Ruffalo, Jeremy Renner and Samuel L Jackson will reprise their roles in Avengers: Age Of Ultron, which is expected to open in UK cinemas in April 2015.
Joss, who has been filming in London, attended the capital's premiere of Marvel's latest film, Guardians Of The Galaxy, with a walking stick, admitting: "My knee's gone out. I'm old."
He added: "I've been wanting to see it. I was working and advising on it early on and then once Avengers got under way, I really couldn't be a part of it any more. I haven't seen much footage or anything. They've been keeping it away from me so I can finish my movie."