9 things you need to know about the big 'Supergirl'/'Flash' crossover:

Different shows. Different networks. Different tones (somewhat, at least). Given all that, could a crossover between CBS's freshman series
Supergirl and The CW’s sophomore hit
The Flash actually work?
After a special press screening, we have an answer: The episode, which airs Monday, is one of the best hours of
Supergirl to date.

or Following the screening, executive producers Andrew Kreisberg and Greg Berlanti fielded questions about the crossover. Here are 9 things we learned:
The Flash fits in Supergirl 's world
Bringing Barry Allen, a.k.a. The Flash, into Supergirl's National City was a gamble — but Grant Gustin and Melissa Benoist got along famously. “Melissa and Grant are so similar as actors, and Barry and Kara both have a light to them. They’re definitely daytime heroes that live more in the light,” Kreisberg explained.
Kara needs Barry
The crossover doesn’t feel like a gimmick. Instead, it plays off the crisis Kara faces when the public turns on her. As Kreisberg puts it, “It felt like The Flash coming to Supergirl really helped Supergirl overcome the challenge that she had been dealing with, and his advice to her about how she should be living her life is about to have repercussions.”
There's romance in the air
Kreisberg is well aware that there have been many “fits and starts” to James (Mehcad Brooks) and Kara's romantic relationship — and when James realizes that Kara and Barry are kindred hero spirits, he gets a little jealous. There's definite movement on the Kara/James front here, though viewers may be surprised to see who's the initiator.
And classic Cat lines aplenty
There’s nothing better that seeing Calista Flockhart chew the scenery — and this episode doesn't disappoint. When Cat meets Barry Allen and sees him standing alongside Kara, James and Winn (Jeremy Jordan), she makes a hilarious comment about a certain network that should get big laughs.
Barry and Kara are facing the perfect villains
Livewire (Brit Morgan) and Siobhan (Italia Ricci), a.k.a. Silver Banshee, are the perfect dastardly duo to fight Kara and Barry. As Kreisberg puts it, Siobhan gave the show an opportunity to introduce a character and keep her around for awhile before the tables turned, "so when [she] transformed, it actually meant something, as opposed to just the crazy thing that happens at the end of Act One.”
Kreisberg says that Supergirl's writers also considered adding Bizarro to the mix, but that idea ended up being “financially irresponsible.”
Get ready for a big race
Getting two superheroes together always leads to arguments about who is stronger, faster or more powerful. So naturally, the writers couldn’t leave that out of this episode. “We wanted to do a shout-out to the classic race,” Kreisberg said.
The crossover will have
Flash repercussions
There won’t be huge storyline shifts based on the crossover, but next Tuesday’s Flash episode — airing, coincidentally, one day after his trip to
Supergirl — will have a reference to the event. You’ll also notice in the Supergirl
episode that Barry wears the Tachyon Drive on his Flash costume.
There will be more crossovers…maybe
After seeing the Supergirl episode, it’s easy to wonder what The Flash or Arrow
would look like if Kara stopped by either one for an episode or two. “We would love that,” Berlanti said. “As storytellers and as fans — and fans of the actors, honestly — we love it when we can figure out a way creatively to do [that].”
There's more fun casting to come
Supergirl has nodded to Superman
history by featuring guest actors like Dean Cain and Helen Shaver — and Berlanti revealed that there are more coming down the pike. Another TV superhero of yore could show up soon, or in Season 2 if the show is renewed: Lynda Carter, who played Wonder Woman on TV in the 1970s. Berlanti hopes to have Carter play the President of the United States.