The CW originally planned to make Arrow episode 2.20 into a back-door pilot for The Flash, bringing back Barry Allen from his two-episode guest spot and showing him after he gains super-speed. The network liked the character so much that they ordered the stand-alone pilot that’s currently in production.
But they’re still establishing background for the Flash spin-off, as two members of the supporting cast are confirmed for Arrow episode 2.19: Danielle Panabaker and Carlos Valdes will appear as Caitlin Snow and Cisco Ramon, scientists based on Killer Frost and Vibe from the comic books. (via FlashFans.org)
Update: THR has more information. Guggenheim explains that bringing in the S.T.A.R. Labs scientists keeps the Flash elements present in Arrow in a much more interesting way than showing Grant Gustin lying in a hospital bed with his eyes closed.
Marc Guggenheim has confirmed that Danielle Panabaker and Carlos Valdes from the #Flash pilot will be in #Arrow 2.19
— Arrow – GreenArrowTV (@GreenArrowTV) March 17, 2014
I am so glad that they are putting Killer Frost and Vibe in the show and hopefully they will show the sad stories on how both of these characters got their powers. And also hopefully they want use Killer Frost as an excuse not to put in Captain Cold.
And lest hope that if the Rouges do show up in the show then they will be their new 52 selves and not have the Guns