[Warning: Spoilers ahead for the Arrow/The Flash crossover.]

Arrow and The Flash have staged mini-crossovers during the past year — Barry, Cisco and Caitlin made their debuts in the second season of Arrow, while Felicity and Oliver have appeared in The Flash's first season. But during the Dec. 2 episode of The Flash and the Dec. 3 episode of Arrow, there will be a full-fledged crossover event between The CW's DC Comics shows.

"If there is anybody who is watching only one of the shows, [this is an opportunity] to sort of check out the other," Arrow and The Flash executive producer Andrew Kreisberg told The Hollywood Reporter on Saturday ahead of a fan-screening of the crossover episodes. "You don't always get that opportunity … but I really feel like there are so many similarities between the shows, as much as there are differences. This was such a great opportunity to give the best of everything we do on both shows: it has great Arrow stunts, it has great Flash humor and heart. We're just really excited."

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Here are seven things to know about the big crossover event.

1. The Flash crossover title, The Flash vs. Arrow is quite literal.

Barry Allen (aka The Flash) and Oliver Queen (aka Arrow) have been allies up to this point, but when Barry becomes determined to prove himself to Oliver, Barry encounters a metahuman whom Flash star Grant Gustin teased "gets in Barry's head, and he gets whammied. Barry's not himself in The Flash vs. Arrow. It was really fun; I got to play a different character, essentially, who was losing his mind. Some truths come to the surface that wouldn't have otherwise, because Barry's been brainwashed, and that's the reason the fight ensues."

Arrow star Stephen Amell acknowledged that "Oliver is keenly aware of [Barry's affliction], and they're fighting, but it's more Oliver trying to get him back under control."

2. The Flash hour will deliver a very big moment for Oliver's storyline.

"There's a really big character, emotional moment for Oliver on The Flash side; [something] that we set up last season and paid off in Flash, and will pay off down the road on Arrow," Kreisberg teased.

When it works its way over to that show will be something fans will leave the crossover event wondering. "I would say, it's not picked up in the Arrow episode [of the crossover]," Arrow executive producer Marc Guggenheim said. "It's like the bomb underneath the table, and I think part of the fun is waiting to see when Oliver is going to learn what the audience learns on Flash. "

3. Team Flash could prove vital to helping Team Arrow with one of their biggest mysteries.

While many of the running plotlines for both shows had to be back-burnered for the crossover event, one key thing will be addressed: Sara's unsolved murder. "Felicity comes to Caitlin for help with the Canary murder," Flash star Danielle Panabaker previewed.

"There is a major step forward in the Canary mystery," Guggenheim teased. "We have the folks at S.T.A.R. Labs in Central City, and they're pretty scientific. They've got a lot of resources at their disposal, and it would be kind of a shame to not have Team Arrow lean on them a little bit to help solve this mystery."

4. Arrow's Laurel is mostly MIA in the crossover … but for a good reason.

Only Arrow characters Oliver, Felicity and Diggle head over to Central City in The Flash hour of the crossover, but Laurel — who is still reeling over the death of her sister and has started her own training — is mostly missing from the Arrow portion of the crossover, too. "Laurel appears in the Arrow episode," Kreisberg said. "She's not as present in these episodes, but that is because we've been building up to her appearance as the Black Canary. We sort of said that Sara was the Canary, but Laurel is the Black Canary. Episodes 10, 11 and 12 are a three-part trilogy that are about her. And episode 13 I think I can spoil, is called 'Canaries.' For fans of Laurel and for fans of Katie [Cassidy], they're going to get more than their fill after the winter break."

5. Arrow's Lyla is in danger.

Diggle has a bit of a lighter turn in The Flash portion of the crossover — nearly all the castmembers present Saturday pointed to David Ramsey's portrayal of Diggle's bafflement at the metahuman powers being one of the crossover's highlights — but things get serious when it's Arrow's hour: his ex-wife/current love, Lyla, is targeted by a former ARGUS agent out for revenge. "Diggle has always been the guy to protect the people he loves, in a human playing field," Ramsey said. "So he doesn't know how to respond [now]. The Flash changes the playing field. We're going to see Diggle embrace The Flash's powers, because he's going to come to need them."

Team Arrow has already been rocked by Sara's death this season, and another person they care about being targeted hits the team hard. "It was crazy, there's been so much violence … anybody can die," Arrow star Emily Bett Rickards acknowledged. "[With life being short] it's not worth it, to be in the clouds, to be above what you're feeling."

6. Team Flash is forced to deal with the reality of how dangerous things can be.

Metahumans and super speed have desensitized Team Flash a bit to the gravity of the situations they face daily, but heading over to the Arrow's Starling City helps put things into perspective for them. "The stakes are more real than [Barry] realized," Amell noted. "Everything in this [Flash] world with metahumans and villains with corny nicknames [is lighter]. Then they come to Starling City, and they experience a life-and-death situation … so that changes things for them a little bit."

"It's actually more interesting to see Cisco accept the reality that this is not a game," Gustin said. "In Central City, our bad guys have powers, and it's easy to have that detachment from reality because I'm running around with lightning speed, and these guys have these other crazy powers, and we're stopping them and putting them in our prison, essentially. Things are just a little more real in Starling, and I think that rocks Cisco a little more. It puts things in perspective for him."

"When the characters cross over from one city to another, they become affected by the tone of that particular city," Flash star Carlos Valdes said. "Starling City has more of a darker, violent edge to it, so when Cisco, Caitlin and Barry come over to that side, there is a change that colors the tone of these characters and the story."

For her part, Caitlin is changed by the experience as well. "She does see the reality," Panabaker said. "It's a little bit easier to sit behind a computer and tell Barry to run faster, than it is when you're faced with a life-or-death situation."

7. The Arrow and The Flash teams would be up for a crossover again … but not quite yet.

It may be a little while before the two shows cross over again this thoroughly, but Guggenheim said, "We'd absolutely love to do it again. It was very hard, but very fun. And we definitely talked about it; we have ideas. I think what's great about how Greg [Berlanti], Andrew and I work is that everything always starts off with, 'Wouldn't it be cool if … ?' A lot of [those] conversations related to the shows crossing over. And what's really great is it looks like with the success of Flash and the success of Arrow, there will be plenty of opportunities coming our way. And we've got some crazy ideas. And the crazy ideas are always the most exciting ones for us."

Twitter: @MarisaRoffman