This Sep. 19th Supergirl panel included Jeremy Jordan, Jesse Rath, Nicole Maines and Helen Slater, with GalaxyCon moderator Patty Hawkins - I only transcribed a few tidbits below...
Galaxycon Live Panel cast of Supergirl 9-19-2020
Galaxycon Alternative1 Sep 20, 2020
-- Jesse: "We're in Vancouver and we're - we're, uh, poised to strike on the new season and, uh, yeah, things are - are exciting, you know. Uh, the ball has just started rolling and we're about ready to start rolling next week. And, uh, that means I'm going to have to shave this manly beard. And so I'm, uh - it's bittersweet."
-- When asked how they got involved in the Arrowverse, Helen said that she received a phone call asking her to appear on Supergirl, adding that a similar thing happened to her with Smallville. She noted that it's become a "trope" to have actors in older superhero shows appear in the newer superhero shows ("legacy casting").
-- When Jeremy auditioned for the Supergirl pilot in 2015, he was initially rejected by the network (CBS), but "Berlanti and folks" convinced the network to let them bring Jeremy on as a guest star for the pilot "as a trial run" and then after the pilot, he was promoted to series regular. He got to do a chemistry read with Melissa Benoist.
-- Jesse was brought in to audition for a character named "Ralph Richards" to appear in one episode. He was already a fan of the Arrowverse shows and was very excited. It wasn't until three or four days before shooting started that he was told he was playing Brainiac-5 and would be painted blue. He found out during his costume fitting and was introduced to the costume fitter as Brainiac-5. He was supposed to be told before then, but they forgot.
-- Nicole auditioned off a self tape. She paid "a kid" $70 to film and edit her audition tape. She didn't hear for awhile, but then she was asked for another self tape, she met the EPs, and then was asked to fly to Vancouver.
-- Jeremy talked about how difficult it was to do the emotional scenes when he left the show. He also praised the Supergirl cast as being "wonderful human beings" and having "no big divas."
-- When asked who messes up their lines the most, Jeremy: "I want to say, a lot of people, but David is really bad at it, Chyler can be pretty epic, and Melissa is notorious for not being able to do act outs. But I think I would say David. Right? Wouldn't it be David?" Jesse: "Maybe. Yeah. I think he features a lot in the blooper reels." Jeremy: "But the thing is, like, he will mess up his line and not even realize he did it."
-- Helen mentioned that she was 18 years old when she played Supergirl in the Supergirl film and that now being brought in to play Supergirl's mom was like "aging gracefully" in this "pretend and play" superhero world.