Originally Posted by Six
Knowing a little bit about CGI works, its pretty obvious that the stand in is simply there for lighting purposes. The stand in itself not actually going to be in the movie. The design however will. And what is up for debate is what character the stand in is for.
If Ultron was in the scene, he'd be in a mocap suit as well. So Ultron could very well be one of the mocapped guys running around. But if that stand in was for Ultron, why would they not have a lighting reference the goons, unless they look exactly the same as him? See where I'm going with this?
Right now, nothing points to it being Ultron, other than the fact that he's metallic.
Also knowing about how CGI works - they are in no way locked into THIS design for the final film - regardless of who it is suppose to be. The very nature of CGI allows them to make radical changes long after these scenes have been shot, limited only by time and money. They also do not "need" a motion capture performance - or at least one shot at this time - if it is Ultron perhaps his mo-cap will be shot later.
Agreed that about all we can assume for sure is that this character is standing in for something shiny and metallic.