While we’re still not entirely sure who Andy Serkis is actually playing on screen in ‘Avengers: Age of Ultron,’ we do know that the actor returned to once again provide some of his excellent motion capture work in the sequel. And though he’s keeping his lips sealed about the role he’ll play in ‘Age of Ultron,’ he has spoken out about some of the motion capture support that went into re-teaming with Marvel’s Avengers.

A while back, rumor had it that Serkis might be playing comic book villain Ulysses Klaw in the film, and while neither Marvel nor Serkis have confirmed that rumor, Serkis did take some time to discuss some of his other acting work in ‘Avengers: Age of Ultron.’ In an interview with Empire, Serkis talked about the motion capture work he did for the villainous Ultron, mostly before James Spader came on:

We did some work on Ultron. On the development of Ultron before James Spader came on board. In terms of movement styles: was he gonna be human-like? Was he going to be robot-like? So we worked with a bunch of different people, from body-popping experts to dancers, to this guy called Neil who’s nearly eight feet tall…

He also worked closely with Mark Ruffalo on Hulk, helping him Hulk out:

We gave Mark weights, we had voice projections so he could do his Hulk roar. On screen we could have a virtual representation of the low-res avatar of The Hulk, so he could come out and feel that sense of scale.

Serkis’ motion capture work is incredible and his process is fascinating, but we’re also pretty excited to see him play an actual role in the film as well—whatever that role may be. And for all the mystery, it doesn’t seem like we’ll find out until the film hits theaters on May 1, 2015.