It could also be something rather small that isn’t really a huge deal, since the Duffer’s love of absolute secrecy has been well-documented throughout the series.
We’re guessing the Stranger Things role(s) in question would specifically reveal some things about Season 3 that the Duffer brothers would rather not be out there.
Unfortunately, the showrunners have already cut off the biggest source of Stranger Things leaks, David Harbour, so we’re forced to wonder just how significant the casting for this secret storyline will be.
New character descriptions for popular shows almost always find their way to the internet, even though they’re often full of doctored information.