Grant Gustin's Barry Allen appeared in two episodes, now several other cast members from the upcoming "Flash" will make their first onscreen appearance on this season's "Arrow" on The CW.
Entitled "The Man Under the Hood," the nineteenth episode will introduce two characters who will be regulars in the spinoff - S.T.A.R. Labs' bioengineering expert Caitlin (Danielle Panabaker) and mechanical engineering genius Cisco (Carlos Valdes).
Executive producer Marc Guggenheim says the appearance is important and came about because the Barry Allen character was left in a coma at the end of the ninth episode of "Arrow" and will stay that way until the pilot episode of "Flash".
A few months ago the plan was to have episode 20 of the series be a 'backdoor pilot' for "Flash", but The CW changed their minds and went with a full pilot instead. That meant some storyline adjustments that were to be done by Barry Allen's character in episode 20 have now been shifted to these characters.
Guggenheim says: "Even though the circumstances of that pitch changed, the idea of bringing in those characters really, really appealed to us because it allowed us to honor our original intention that we had at the beginning of the year, which was to do something Flash-related around episode 19 or 20 without of course having comatose Barry and basically paying Grant Gustin a lot of money to sit around with his eyes closed unconscious the whole episode. It allows us to further flush out The Flash universe in Arrow, we get to see their dynamic."
Source: The Live Feed