Before it had a spot on The CW's fall 2014 schedule, "The Flash" star Grant Gustin portrayed soon-to-be-speedster Barry Allen on two "Arrow" episodes late last year. The connections didn't stop there -- Carlos Valdes and Danielle Panabaker inaugurated their "Flash" roles as Cisco Ramon and Caitlin Snow in an "Arrow" installment this past spring -- and look to grow even deeper when both shows are up and running.

"For us, so much of the fun of getting to do a spinoff, and getting to do these superheroes, is that they do exist in the same world, and that we do get to see them interact," Andrew Kreisberg, an executive producer on both "Arrow" and "The Flash," told a group of reporters including CBR News during roundtable interviews late last month at Comic-Con International in San Diego. "The big crossovers, like having Oliver and Felicity and Dig coming over to 'Flash,' but then just even the little things, like the scene in the pilot. On a news screen on a 'Flash' episode the newscaster says, 'In other news, Ray Palmer's takeover of Queen Consolidated...' just reminding people that they are in the same world. Not in a way that if you're not watching one of the shows, you're going to be like, 'What the hell is going on?' but if you are watching both shows, it's like, 'Oh, that's so nice.'"

It's already been revealed that "Arrow" star Stephen Amell has an appearance in the "Flash" pilot, and it's also been confirmed that Emily Bett Rickards will appear as Felicity Smoak in the junior series' fourth episode. It's a significant guest appearance, as Gustin's two "Arrow" episodes established a possible romance between his and Rickards' characters -- before Barry was struck by lightning that put him in a coma (but on the plus side, also gave him superpowers).

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"There's unfinished business with Barry and Felicity," Kreisberg said. "I think it would have been a disservice to all the people who were supporting 'Barricity' -- and as much as it clearly had meant to Felicity over the back half of the season -- to not address that. In some ways, it was more important to have Felicity and Barry crossover than it was to do bigger crossovers, like the Arrow and the Flash."

The fourth episode of "The Flash" will also introduce actor Wentworth Miller as Leonard Snart, known in DC Comics lore as Captain Cold -- one of the foremost figures of The Flash's "rogues," and a personal favorite of "Flash" TV executive producer (and DC chief creative officer/former "The Flash" comic book writer) Geoff Johns.

"Captain Cold is the Joker, he's Lex Luthor, for the Flash," Kreisberg said. "We seem to always be the beneficiary of great timing. Wentworth, since he finished 'Prison Break,' he's been screenwriting. He was just now sort of wanting to dip his toe back into acting. It was perfect timing."

Kreisberg said he believes The Flash has "hands-down the best villains" of any superhero -- with the admitted possible exceptions of Batman and Spider-Man -- which helps open up opportunities that don't necessarily present themselves on "Arrow."

"The villains are in some ways just as interesting as the hero," Kreisberg said. "Not because they're flamboyant, but because they have their own lives. Getting to dip into them -- it's no secret Green Arrow doesn't have the world's greatest villains. After Merlyn and Vertigo, you're scraping the bottom of the barrel. But there's this sort of endless supply of villains we're going to get to use [on 'The Flash']."

Even though "Arrow" and "The Flash" are firmly set in the same world, the inherent differences between the two characters provide the creative teams with a wide array of material to explore in the newer show -- both the grounded world of "Arrow" and the more fantastic situations of "The Flash."

"We're actually going to have one villain from 'Arrow' come over in one of the early episodes," Kreisberg said. "What's so much fun about 'Flash' is -- on 'Arrow,' we've generally steered towards the non-powered villains. Even when we had some of the powered villains, we de-powered them, or we fit them into our world, like the Royal Flush Gang. Now, doing 'The Flash,' it's opened up -- not only the Flash, who has the best villains of any comic book characters, but also a lot of the other superpowered villains in the DC Universe. We're really excited to be exploring all of them, but also having the opportunity to revisit some of the Arrow's nemeses, and bring them over."

Of course, like all classic comic book characters, Flash's villains are a product of their time, and in some instances may not exactly translate to a prime-time viewing audience. It's something "The Flash" crew is aware of, though it sounds like they're not going to disown their comic book roots.

"Sometimes they have not the greatest names -- like Rainbow Raider, or even Captain Cold," Kreisberg said. "Cisco, he's sort of a fanboy on this show, he's constantly coming up with the names for people. As he's doing it, he'll pitch a name, and they'll be like, 'that's really lame.' But it's a way to namecheck some of these villains that I think the fans will find fun."

"Fun" is a word that those involved with "The Flash" keep coming back to when describing the series. Greg Berlanti, also an executive producer and co-showrunner on both "Flash" and "Arrow," making a lighter, brighter superhero show was absolutely the goal.

"It was definitely very deliberate," Berlanti told CBR News. "You never know how well you're going to pull it off, but definitely the intent was deliberate -- in large part because it reflected whose Barry's character was. In the comic book, here was a guy who could run really fast, and he's dealing in the sci-fi world. He had a sense of humor, and a sense of awe of the things that were happening to him, and around him. You put that all together, and you just get a brighter show. And we wanted to contrast it from 'Arrow' -- and we were all big Donner 'Superman' fans. I've always felt that was the most iconic version of the bright superhero world."

It's an especially meaningful element for Gustin, who said he finds the tone especially well-suited for what he brings to the title character.

"It's the only reason I'm playing the role," he said to CBR. "I think if it was a darker show, it wouldn't be me. I'm loving it. I'm having more fun than I've ever had -- hopefully that comes through. We love it, we love each other, I think that makes the chemistry on-screen better."

While Gustin is in his first starring role, after appearances on "Glee" and "90210," fellow "Flash" cast member Tom Cavanagh, who plays Dr. Harrison Wells of S.T.A.R. Labs is a TV veteran -- who also finds the sunnier outlook of "The Flash" to be refreshing.

"One of the cards that we have in our back pocket, this is extremely beneficial for us -- most hour longs don't have this lightheartedness," he said. "We have all the stuff, but that is like a mandate from the guys to bring that again and again and again. I love that part of it. I love the fact that we have these moments of levity, as well as all the action, as well as the stories that we have. I think a lot of that comes from the fact that sometimes a lot of superheroes are sitting with cowls, and brooding up above -- whereas we've got Barry Allen, who's like 'Whoa!' He's very innocent, a very regular person, and it's appealing."

"The Flash" is scheduled to debut Oct. 7 on The CW.