Mo-cap technology has become incredibly valuable on productions like Avengers: Age of Ultron, in which Andy Serkis collaborated with Mark Ruffalo and James Spader to bring the Hulk and Ultron to life. You really couldn’t ask for a better performer to help with your mo-cap performance. In a new interview, Joss Whedon opens up about what it was like to work with Spader and the actor’s difficulties with the technology, which still yielded some impressive results.

Whedon sat down with Marvel in the new video interview, where he’s asked specifically about Spader’s performance and working with the actor, known for his particularly unnerving roles. “Evil sentient James Spader” is a great way to describe the actor, but even better might be Whedon’s impression, which is an almost spot-on imitation of Spader’s signature cadence.

There are so many exciting aspects of Age of Ultron, including Vision’s debut and super-powered twins Scarlet Witch and Quicksilver, but the villainous Ultron himself is particularly intriguing, offering the most terrifying threat the super-team has faced to date. In the trailers, Ultron looks pretty remarkable, but with the way the villain is uniquely constructed (and somewhat deconstructed), the use of mo-cap was necessary—and the end results look awesome.

Whedon goes on to discuss the difficulties Spader had with the mo-cap technology, but if he had any trouble giving that performance, it doesn’t show.

In the May 1, 2015 release, when Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry and Earth’s Mightiest Heroes, including Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow and Hawkeye, are put to the ultimate test as the fate of the planet hangs in the balance. As the villainous Ultron emerges, it is up to The Avengers to stop him from enacting his terrible plans, and soon uneasy alliances and unexpected action pave the way for an epic and unique global adventure.

Avengers: Age of Ultron hits theaters on May 1.

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