Suicide Squad is the first film in the DC Extended Universe that was not directed by Zack Snyder.
If you’ve already seen Suicide Squad, or don’t care about spoilers, then you know that early in the film, right after we are first introduced to Captain Boomerang, we meet the guy who caught him, The Flash.
In addition to acting as an executive producer, Snyder also directed one scene, the one where The Flash makes an appearance.
If Suicide Squad had just left well enough alone, and not released one final trailer the week before the movie came out, the early appearance by Ezra Miller as The Flash likely would have stayed hidden.