As you would if you were him, Zack Snyder posted a photo of himself on the Justice League set and wearing a Batman gauntlet to Twitter, causing nerds everywhere to scream, “WHAT’S ON HIS TABLET?” The answer is Deathstroke. Deathstroke is on his tablet.
Source: io9 – Zack Snyder Reveals a Very Tiny Hint About Justice League’s Deathstroke