John Barrowman has been a part of The CW's DC TV universe almost since its inception, having first appeared as Oliver Queen's arch-nemesis Malcolm Merlyn in season 1 of Arrow. Since then, the Arrowverse has expanded to include fellow DC shows The Flash and Legends of Tomorrow, and starting next season will include the newly-relocated Supergirl as well.
True to their comic book roots, the shows comprising the DC TV universe have featured plenty of crossovers over the last few seasons, with the Green Arrow appearing in The Flash, The Flash appearing in Arrow, and episodes of The Flash leading directly into Legends of Tomorrow. Supergirl got in on the action even before its move away from CBS, with Supergirl and The Flash meeting across universes and bonding over their similarly cheery demeanors. But where does all that crossing over leave Barrowman?
According to TV Line, it leaves him with a lucrative contract to appear as a series regular not only on his home turf in Arrow, but also in every other DC show on The CW. That means that Malcolm Merlyn has the potential to pop into any episode of The Flash, Legends of Tomorrow, or Supergirl and cause trouble for their respective heroes. It's all part of the continued effort to make all the shows part of one seamless universe (or two universes, technically, with Supergirl taking place in a separate one). TV Line points to a comment from producer Greg Berlanti about a similar deal made with actor Wentworth Miller (Captain Cold) back in May, but which is also applicable to Barrowman's contract as well:
"What it really emulates to us is the comic books themselves, where there really are a cast of characters. In success, we hope to continue with other characters finding their way across all the shows."
Production on season 5 of Arrow recently began in Vancouver, with the first episode of the season revealed to be titled "Legacy." Series star (and recent star of Teenage Mutant Ninja Turtles: Out of the Shadows) Stephen Amell has promised that the fifth season will "refocus the core mission of the show" and teased that the "Flashpoint" plotline from its sister show The Flash will have a major effect on the season as well. How that may lead to Merlyn appearing across multiple shows is not clear, but the Flashpoint Paradox events in the comics and standalone animated movie led to a radically changed DC universe. With that being the case, any number of alternate universe scenarios for Merlyn are possible.
Juggling the duties of writing a TV show as a standalone entity and as part of a shared universe is no easy task. One of the chief complaints about both DC and Marvel comics over the years has been how difficult it can be to follow storylines as they take place across multiple titles, and the showrunners for The CW's DC universe have to be careful not to fall into the same trap. For the shows' actors, though, the continued crossovers at the very least offer the chance to make more appearances than they would otherwise, and with more appearances comes more money.
Arrow season 5 is scheduled to premiere on Wednesday, October 5, 2016 at 8PM ET; The Flash season 3 is slated to air on Tuesday, October 4, 2016 at 8PM ET; DC’s Legends of Tomorrow will return on Thursday, October 13, 2016 at 8PM ET, Supergirl will make its season 2 debut on The CW on Monday, October 10, 2016 also at 8PM ET.
Source: TV Line