A video, which Amell posted to Twitter, shows off chairs from the set. Each of the chairs features the name of a cast member, including Emily Bett Rickards, David Ramsey, Amell himself, Carlos Valdes, Danielle Panabaker, Melissa Benoist and Grant Gustin. In between Benoist's and Gustin's chairs, however, there is an unlabeled chair. When passing this chair, Amell says, "secret."
Judging by the logos present on each chair, this video was probably taken on the set of Arrow. It's unclear if the blank chair belongs to someone we already know is joining the crossover, such as Ruby Rose, Cassandra Jean Amell or Elizabeth Tulloch, or an entirely new cast member who has not yet been announced.
Tyler Hoechlin, who will reprise his role from Supergirl Season 2, is also set to be part of the crossover, which will introduce Batwoman, Nora Fries and Lois Lane.
"Elseworlds," this year’s Arrowverse crossover event, will begin with The Flash on Sunday, Dec. 9 at 8 pm ET, continue with Arrow on Monday, Dec. 10 at 8 pm ET and end with Supergirl on Tuesday, Dec. 11 at 8 pm ET.