Joss Whedon says Ultron is ''everything a robot never is''.

The 49-year-old director is working on 'Avengers: Age of Ultron' and he's been having a ''blast'' developing the character which is played by James Spader.

He explained to Empire magazine: ''I'm having a blast with Ultron.

''He's not a creature of logic - he's a robot who's genuinely disturbed. 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.''

Spader joins an A-List cast including Robert Downey (Iron Man),

Chris Hemsworth (Thor), Mark Ruffalo (The Hulk), Chris Evans (Captain America), Scarlett Johansson (Black Widow), and Samuel L. Jackson (Nick Fury).

Although there a number of big names Whedon insists every character in the forthcoming epic has a significant role to play.

He explained: ''I fiercely dislike the idea of just throwing in more people for the sake of doing that.

''But last time I had all of Earth's Mightiest Heroes versus one British character actor, and I needed more conflict.''

The filmmaker wants to bring a different aesthetic to the picture to make sure it isn't too similar to 2002's 'The Avengers'.

He added: ''The number of different looks and textures and moods we're getting from the British locations is stupid awesome, 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.''