Stephen and Robbie Amell, who play as Firestorm and Green arrow, in the CW 'Flarrowverse' are, in case you didn't know, are cousins. this, although not a big deal, is, within itself, pretty cool, that they get to work together like that.
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half an hour ago, @amellywood (Stephen) posted this:
Please enjoy #TheFlash tonight. Acting w @RobbieAmell was phenomenal. It was the first time. Won't be the last. @grantgust (your cool too)
I don't know about you, but to me, that is a teaser. Won't be the last. that's some pretty teaserish material, saying it won't be the last. Now, there are a couple things that Stephen could be saying, but a few make more sense than others... for example: In case you didn't know, Robbie was the main character in a T.V. series: The Tomorrow People. This show was also by the CW, but it flopped within the first three episodes, and afterward they lost viewership, and the show was cancelled after season 1. Since Tomorrow people plopped (by the way, some of the other characters from that show have been on TV since such as Peyton List as golden glider, and Luke Mitchell as an Inhuman) I think Robbie would be more inclined to pursue his movie career, maybe even in TMNT 2, along with Stephen, explaining the tweet.
Another possible explanation is a more obvious one: Firestorm on Arrow. I am not going to delve too deep into this one, because there are conspiracies and that is not really my thing, but I will say this: that would be epic.