It seems like the issue that has come up with movies adapted from comic book source material is the "Cinematic Universe". So far, Marvel has been good at keeping its characters from the Avengers in continuity that spans several films, as well as the Netflix shows like Daredevil, Jessica Jones, and more, even staying within the continuity on TV with Marvel's Agents of S.H.I.E.L.D. and Agent Carter.
It seems like DC comics, Marvel's number one competitor, has been lagging behind, but Batman v. Superman: Dawn of Justice will change all that as Batman and Superman will team up with Wonder Woman, and this will lead to two Justice League movies as well as other various movie projects. However, how does this Cinematic Universe continuity work with DC's popular shows like Arrow, The Flash, as well as the new Supergirl and the coming DC's Legends of Tomorrow?
It is quite possible that Steven Amell, star of Arrow, might have spoken out of turn when someone at a convention asked him if it was possible to see a high-profile Batman villain appear on Arrow, like the Joker. He gave his "short answer" of yes, as in all these characters exist in the DC Universe.
According to Screen Rant, he also had a longer answer: "But of course, The Joker could come to Arrow, Batman could come to Arrow, Superman could show up on The Flash. There's a whole myriad of options because again DC and WB have a slate of films coming up, and those films happen, and maybe Arrow's still around when they're done. Or maybe there's a change and people are just like, 'You know what, let's invest in something that's doing well for us.' So you never know."
Arrow has had his visit from the Batman villain Ra's al Ghul, so it wouldn't be too far a stretch to have the Joker show up. The only problem is that the Joker is booked as he is appearing in DC's big-screen Suicide Squad film, and possibly in a solo Batman film project. It seems pretty obvious that DC's movies do not take place within the same continuity as the TV universe established in Arrow or The Flash, also known as "The Arrowverse."
In fact, it is not even known whether or not this Arrowverse even exists in the same continuity as Supergirl. Considering that Arrow and The Flash are on CW and Supergirl on CBS, it is often difficult to get these elements to line up when different studios could own the rights to comic book characters. There are rumors that The Flash could cross over into Supergirl's world, which would directly tie the entire universe together well. As it is, Superman has appeared on Supergirl, but in an odd way where his face is never shown.
For DC to begin production on Arrow, have a hit show, and then not tie-in to its movies seems an odd play. The average viewer might enjoy jumping on the continuity bandwagon that Marvel has had with its Cinematic Universe, but could be easily confused that the Flash in the Justice League movies is different than the one on TV.
There is a way to fix this. Next year, the CW will bring DC's Legends of Tomorrow to television, and it is a big budget series that brings in a lot of characters who have appeared on Arrow and The Flash. The show will feature Rip Hunter, and he will play a time traveler.
Here's what could happen: if time travel is introduced in any plotline, then there is always an easy way to change things up, as far as the whole universal continuity is concerned. DC has had several big events in its history like Crisis on Infinite Earths, Zero Hour, and other massive events that crossed over into all their titles and changed the continuity of the entire universe. What could happen is that events on DC's Legends of Tomorrow done in the past could alter the present, so that all of the events on Arrow, The Flash, Supergirl, and even the DC movies could tie together. It is something to think about.