When asked why Grant Gustin, TV's Flash, wasn't going to be The Flash in the DC Movieverse, Zack Snyder gave an answer that illuminated two things: the fact that DC is consciously building a multiverse and the limitations of the "filmmaker-driven" approach to the DC movies.
“Even if Grant Gustin is my favorite guy in the world and he’s very good, we made a commitment to the multi-verse (idea), so it’s just not a thing that’s possible.”
That, including the dash in the word multiverse, is from the New York Daily News. The idea that there's a DC multiverse has been pretty obvious since this season of The Flash has been dealing with just such a thing, but whether or not the movies were aware of it was another matter altogether. After all, the episode where Flash travels between Earths and sees other worlds passing him by did not include any sort of Batman or Superman stuff - only previous and concurrent TV shows. Yeah, it was saying, there's a multiverse... but it's confined to TV. Snyder acknowledging the multiverse (coupled with some stuff that happens in this week's Batman v Superman) means we could one day see Crisis on Infinite Earths as WB's version of Avengers: Infinity War.
So separate but equal. That's fine. But why must the TV Flash be so separate? Tone, of course.
“I just don’t think it was a good fit,” Snyder told the Daily News. "I’m very strict with this universe and I just don’t see a version where...that (tone is) not our world.”
This is an interesting statement coming from the guy who has said that the DC Movieverse is going to be filmmaker driven. That statement is intended to throw shade at the Marvel Cinematic Universe, which is often derided as a corporate committee creation. The DC films, we are meant to infer, will be where artists can spread their wings.
As long as their wings fit within the tonal parameters established by Zack Snyder.
This is no surprise - of course all of the DC movies are going to have the darker tone established in Man of Steel - it's their main selling point - but I do feel like this statement underlines just how tonally similar these movies will be. I actually don't think that the tone of The Flash is egregious wacky; while I understand that there are a lot of storytelling and continuity issues that would be created by joining the TV show and the movies I don't think tone would be that much of a bridge to cross. Snyder clearly disagrees, which I think gives some insight into just what kind of a tone we can expect from the next five years of DC films.