A new image of McNairy has since been released online (via Batman-News),
and has sparked a bit of conversation in the fan community.
Here's the photo:
McNairy is wearing green screen capture material on the bottom half of his legs. Speculating has led some fans to believe this could indicate McNairy as playing The Flash in the film, and that the green leggings are so that they can make his legs move at super speed in post-production.
Possible, but it seems like if that were the case they'd probably want his entire legs to go at the same speed, rather than just the part below the knee.
There's also the possibility that the green is there so they can add some kind of speed force special effect to his movements - lightning or blur - but green screening that much of McNairy's legs seems an odd way of going about that kind of effect.
Being that it's green screen, there's a multitude of possibilities. Practically anything can be added in post-production. McNairy could end up with cyborg legs, or his legs could be on fire, or any number of other things. Point being, it's hardly conclusive.
Batman v. Superman: Dawn of Justice opens March 2, 2016.