Brian Truitt

USA TODAY

When the Flash meets Green Arrow on a CW crossover, there’s usually a clash of egos and tension between the super bros. And on the big screen, Batman and Superman aren’t getting along, either.

But when the Flash hangs out with Supergirl, there’s no fisticuffs, only fun.

“Isn’t that nice?!” says Melissa Benoist, the actress clad in the cape and “S” shield. “It’s not what we’re used to seeing.”

CBS'Supergirl gets a visit Monday (8 ET/PT) from The Flash's Grant Gustin: His Barry Allen, trying to get faster in order to take on arch-villain Zoom, gets too speedy on his dimension-hopping tachyon device and winds up on the parallel Earth of Superman and his cousin, Kara Danvers (Benoist).

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They even have a meet-cute moment in National City when he goes to save a falling woman, who turns out to be Kara, and she flies off in her civvies.

Even though he deals with metahumans weekly, “Barry hasn’t seen anything quite like Kara,” Gustin says. “But they’re also both slightly socially awkward, goodhearted people, and it’s not too hard for them to get along.”

Flash helps Supergirl take on the dastardly duo of Livewire (Brit Morgan) and Silver Banshee (Italia Ricci), and in return Kara helps Barry get back to his Earth.

CW's The Flash also comes along at just the right time for Supergirl, both in terms of ratings — the first-year show is still waiting for renewal — and its central superheroine. An incident with red Kryptonite turned Supergirl into a bad girl and a rampaging public nuisance, and the Scarlet Speedster has to help guide her on the quest to win back the hearts and minds of National City, says Andrew Kreisberg, an executive producer of both series.

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Much like the Green Arrow (Stephen Amell) was to Flash, Barry acts as an older, wiser mentor figure for Supergirl, Kreisberg says. “Kara is having an existential crisis that revolves around the fact that she’s a brand new hero. Barry is able to say to her, ‘You’re just starting out, but what you need now is time.’ ”

In turn, the Flash finds her inspiring, Benoist says. “She’s all chutzpah and enthusiasm.”

Arrow and The Flash have had two crossovers in as many seasons, but Benoist didn’t think a Flash/Supergirl combo would work logistically because they air on different networks. However, because all the higher-ups were on board — and CBS owns half of CW — “it turned out to be a little bit easier than people think,” says Kreisberg.

Gustin and Benoist already knew each from Glee, and she dropped the crossover news on him backstage at the Golden Globes in January. “We were both pretty giddy about it,” Gustin recalls, “more so than the fact that we were at the Globes.”

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The excitement hit a superpowered level when they finally shared their first scene together in costume, Benoist with her signature Super crest and Grant with the familiar Flash lightning bolt. It encapsulated “the joy of what people love so much about comic books and superheroes,” says Benoist.

“I knew how big of a deal it would be for fans," Gustin adds, "but that’s really when it really hit me how cool it would be for everybody.”