Grant Gustin's final crossover happened without his knowledge.
The Flash star appeared in the April 20th episode of Titans, titled "Dude, Where's My Gar?" As Beast Boy (Ryan Potter) travels through The Red — like the Speed Force, but it connects all living organisms and gives him his power — he sees different DC Comics figures, including Brec Bassinger from Stargirl, real-life comic writer Grant Morrison, and Barry Allen (Gustin) running in his supersuit.
The CW Grant Gustin on 'The Flash'
But when asked about how he showed up for that crossover moment during his series finale EW cover story shoot, a shocked Gustin asks, "I did? When?" The actor laughs as his cameo in that scene is explained to him. "This is good. Don't cut this," Grant says, adding, "It's old footage, obviously. Wow. That's really cool. I had no idea."
Gustin previously thought Stephen Amell's final appearance as Oliver Queen a.k.a. the Green Arrow in the final season of The Flash was going to be his last crossover moment. "I knew pretty early on, before we started filming the season, that we were going to have Stephen," Gustin says. "I think Stephen told me first, just because he was really quick — [showrunner] Eric [Wallace] texted me five minutes later or something."
Getting one last episode with Amell before The Flash — and the Arrowverse — ended for good was something that Gustin appreciated, because the entire interconnected universe really started with the two of them.
"Stephen and I, that was one of the most special relationships for fans to see on camera," Gustin says. "And we also just had so much fun. We're really different guys and actors, but we also have a lot of things in common, interest-wise, and we always had fun working together and our characters were so opposite that it was just fun to play off of each other. It was exciting to have Stephen back."
The Flash series finale airs Wednesday, May 24, at 8 p.m. ET/PT on the CW.