In previous annual crossover episodes, CW’s resident superheroes have tussled with invading aliens and evil Nazi doppelgangers. This year, they’re going full “Freaky Friday.”

The three-part “Elseworlds” storyline – kicking off on Sunday’s “The Flash” (8 EST/PST) and continuing on Monday’s “Arrow” (8 EST/PST) and Tuesday’s “Supergirl” (8 EST/PST) – body-swaps two of the network’s central heroes, Green Arrow, aka Oliver Queen (Stephen Amell) and the Flash, aka Barry Allen (Grant Gustin). After they mysteriously wake up one day living the other’s life and brandishing the other’s skills, the two guys reach out to their powerful pal Supergirl (Melissa Benoist), meet her iconic cousin Superman (Tyler Hoechlin) and head to Gotham City on their path to setting things right in these altered worlds.

Batman’s not in town, but Batwoman is: Ruby Rose makes her debut as Gotham’s female caped crusader before getting her own potential series next year on CW. (The first crossover similarly spun “The Flash” out of “Arrow.)

After seven years on "Arrow," producers gave the DC bunch free rein, essentially telling them "You can have whatever you want," Amell says. "Which I don't think has ever happened before. Bottom line, this was the most fun that I've ever had, probably by design."

Cast and creators break down what those hero lovers can expect from “Elseworlds”:

Oliver and Barry walk a mile in each other’s super-shoes.

The heroes share a mutual respect, but to get out of their predicament, they also have to understand each other, says executive producer Marc Guggenheim. “To use their new powers and skills, they basically have to channel the other's personality.” And Supergirl acts as a mediator “to police the two boys,” Benoist says. “It's not always peachy between them.”

Gustin really loved being Green Arrow.

Not only did he feel like an emerald-tinged warrior, “it was also pretty cozy compared to the Flash suit. It's got a hoodie, which is awesome,” says Gustin, unaccustomed to a quiver on his back: “I would get caught in doorways.” He admits he got a 30-minute lesson in archery to perfect his form and he only shot computer-animated arrows. “I don't think Stephen shoots real arrows either, to be honest. When he sees that quote, he'll be like, ‘Nope, there has been this time and this time...’ ”

Batwoman is a lady of multiple personas.

Like her cousin Bruce Wayne, Rose’s masked Gotham City heroine is an enigma of the night who’s fierce, uses a lot of gadgets and can kick serious posterior, says Caroline Dries, who wrote Batwoman’s introductory episode and is heading the new show. The vigilante’s alter ego, Kate Kane, “is much more elusive and mysterious.” While she isn’t interested in having a new bunch of superfriends, Kate shares a warm moment with Supergirl: “They're both cousins of very famous heroes, and Kara sees a lot of herself in Kate,” says Benoist.

New towns have old connections.

The “Arrowverse” visits Smallville for the first time, resurrecting the ranch in Vancouver from the old WB series. “It was so picturesque and very Americana and it felt so Superman,” Benoist says. And to replicate Gotham, the shows filmed in Chicago – the same setting Christopher Nolan employed for his “Dark Knight” trilogy. “We're really establishing this sort of dead city,” Dries says, “the idea that something terrible has happened in the wake of Batman's absence and it’s been overrun with crime and despair.”

Green Arrow, meet Superman.

Being introduced to the Man of Steel “is one of the funniest interactions in the crossover for me,” Amell says. “Oliver has no problems with nor is he intimidated by Superman.” But Superman faces an intimidating opponent in the black-suited Dark Superman (Hoechlin) during the crossover. Adds Benoist: “It's unnerving to see someone in that suit, wearing that (shield), doing the things that he was doing.”

The crossover has lasting consequences.

Unlike the previous multi-episode affairs, “Elseworlds" affects the characters of each show going forward, and Benoist reveals it will carry into the 2019 crossover. Amell says it’s leading somewhere, “and if you're a big comic-book fan, you'll be able to at least theorize about where that could be.” Guggenheim advises viewers to pay attention to the last line of dialogue: “We drop the hammer and it’s really cool.”