"The Avengers: Age of Ultron" is currently being assembled in the U.K., and writer-director Joss Whedon took a moment to bring fans up to speed on the Marvel sequel.

In addition to Iron Man, Hulk, Thor, Captain America, Black Widow, Nick Fury and what's left of S.H.I.E.L.D., "Ultron" also features new Marvel heroes and villains such as the Scarlet Witch (Elizabeth Olsen), Quicksilver (Aaron Taylor-Johnson), Baron Wolfgang von Strucker (Thomas Kretschmann), the Vision (Paul Bettany), and, of course, Ultron, played by James Spader.

It's a lot of characters to keep track of, but Whedon sees them all as necessary to the sequel's story. 

"I fiercely dislike the idea of just throwing in more people for the sake of doing that," he told Empire. "But last time I had all of Earth's Mightiest Heroes versus one British character actor, and I needed more conflict."

Whedon also revealed a bit more about the title 'bot. "I'm having a blast with Ultron. He's not a creature of logic - he's a robot who's genuinely disturbed," he revealed. "We're finding out what makes him menacing and at the same time endearing and funny and strange and unexpected, and everything a robot never is."

Whedon also mentioned the look of the new film, which is currently shooting on location in the U.K. "The number of different looks and textures and moods we're getting from the British locations is stupid awesome," he said, "because this, palette-wise, is very different. I'm trying to make a different film. Because why would you make one movie twice? That seems weird."

"Ultron" also stars Marvel mainstays Robert Downey Jr., Mark Ruffalo, Chris Evans, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Samuel L. Jackson, Cobie Smulders and Don Cheadle. 

See Downey Jr. and Ruffalo's on-set photos here.

"The Avengers: Age of Ultron" opens in the U.S. on May 1, 2015.