A significant portion of the villains in The Suicide Squad have already appeared in live-action, thanks in large part to the Arrowverse. Even though the movie is being noted for utilizing C and D-list DC Comics villains most audiences have never heard of, this isn’t actually the first time some of these characters have been used outside of the comics and the cartoons.
Prior to their roles in James Gunn’s The Suicide Squad, six of the movie’s characters made appearances in shows set in the Arrowverse. Long before being played by Idris Elba as a central character in a DC movie, Bloodsport was a one-off adversary in Supergirl’s season 3 premiere. King Shark and Captain Boomerang were recurring villains on The Flash and Arrow respectively. Double Down, who had a cameo in The Suicide Squad's prison scene, is another villain who was once featured on Arrow. Played by Cynthia Addai-Robinson, Amanda Waller was the leader of the Arrowverse’s Suicide Squad and a major figure in Oliver Queen’s origin story through the show’s first four seasons. Lastly, The Flash used the original incarnation of The Thinker as its main villain in season 4.
The Arrowverse and The Suicide Squad sharing so many characters can be explained by their obscure status in DC Comics. It’s no secret that getting permission to use A-list DC superheroes and villains can be quite problematic for the Arrowverse. Usually, DC having plans to insert a particular character in a film means they’re off the table for shows like Arrow and The Flash. So, while Superman and the Flash aren’t off-limits, characters like Green Lantern, Wonder Woman, Batman, and more will likely remain unavailable. The same basic logic applies to the iconic villains as well. For these reasons, it’s easier for these shows to work with lesser-known characters where the rules about how they can be used are much less restrictive. Bloodsport, King Shark, Captain Boomerang, and the Thinker all fit that bill at the time of their Arrowverse appearances.
The Arrowverse having to rely on unknown DC Comics characters, and James Gunn making a conscious decision to go down this road for The Suicide Squad, explains why there’s so much overlap. Because of this, some viewers may not have had any familiarity with Ratcatcher 2, Polka-Dot Man, Weasel, or Javelin, but they could easily remember King Shark as the misunderstood metahuman that battled Barry Allen or the Thinker as the genius college professor who schemed to steal the powers of other characters on The Flash.
It’s worth noting that this aspect of the Suicide Squad team in general applied to the first movie as well. Some of its characters (who weren’t in Gunn’s movie), such as Katana and Deadshot, appeared on Arrow. The Suicide Squad roster being loaded with obscure names in DC Comics was also a boon to the show that preceded the Arrowverse, Smallville. The Superman prequel series had its own versions of Rick Flag, Amanda Waller, and more. However, Gunn’s The Suicide Squad elevating so many of these characters beyond their D-list reputations may mean that drawing from their corner of the DC Universe is a luxury that the Arrowverse and other DC shows will no longer have in the years that follow.