That Suicide Squad trailer is so fun, right? Dark, but exciting and funny? Well, apparently, that's not the movie director David Ayer made. He made something much more serious and, after the success of the trailer, rumour has it reshoots are taking place to add some levity to the movie. Birth Movies Death says the reason for the reshoots are that "Every joke in the movie is in [the above] trailer". After the some criticism of the "heart attack serious" tone of Batman v Superman, the reshoots will hopefully give audiences the film that the awesome trailer is selling.
According to BMD's source, these aren't just mere reshoots. Most films schedule reshoots later in the process for simple clarity things (even Star Wars: The Force Awakens was reshooting and adding things weeks before opening). No, these reshoots are apparently "big -- tens of millions of dollars big". That doesn't necessarily mean there are major explosions or anything either. Apparently they're just "beefing up fun character moments and interactions". That can get expensive when a lot of work, and actors, have to be put together quickly.
On the other hand, another well-connected journalist, Umberto Gonzalez of Heroic Hollywood, points out that the reshoots are "customary practice". So just the usual stuff, not the major ones BMD are reporting.
Either way, if there's a way to improve the film in any way before its release in August, all the better. No one should look at this negatively. If true, it should be viewed as a studio that, no matter what the scope, is putting more money into a movie to make it successful.
We contacted Warner Bros. for confirmation on this but haven't heard back as of writing. The studio generally generally doesn't comment on rumours.