DC is letting some info slip from the set. This is all planned (there were multiple press there). I don't recall this from BvS, is this new behavior from WB? Being more open, and letting more news flow from the set?
Here's someone talking about the clip they showed to the reporters (I don't think the clip is online anywhere):
comicbook.com/2016/06/21/barry-allen-mee...from-justice-league/
But there was more:
comicbook.com/2016/06/21/everything-weve...ustice-league-movie/
Including Affleck talking about there being more humor in the movie.
really, we wont' know anything until we see the movie. But it does lead back to the other stories we're hearing. I don't know what the REAL plan was all along, but I am skeptical that Synder's original plan was to get lighter in the JL movie. BvS was darker than MoS -- it was a purposeful choice, and you could have started to go lighter in it, by writing a different movie. It didn't HAVE to be BvS. I will note that Goyer isn't writing JL.
The real key is "have to see the movie first".