The shared universe between The CW superhero shows “Arrow” and “The Flash” has been teased and was inevitable since before “The Flash” even became a real show, with Barry Allen first showing up on an episode of “Arrow” last season.

But now the show is real — and a mega-hit — and so is a real crossover event, and at a special screening of the two episodes on Saturday, cast members and showrunners teased how it came together and what ramifications the episodes will have for the heroes of both shows.

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“The Flash” kicks things off with Team Arrow – Oliver Queen (Stephen Amell), John Diggle (David Ramsey) and Felicity Smoak (Emily Bett Rickards) — landing in Central City as characters from both shows team up try to track down a rage-inducing meta-human.

And as the episode is aptly titled “Flash vs Arrow,” it will pit Oliver Queen against Barry Allen (Grant Gustin) in a head-to-head superhero showdown that’s been in the works for a long time.

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“We’re all fans, and we’ve all sat around debating who would win in a fight, like Batman or Superman or Wolverine,” said Andrew Kreisberg, an executive producer on both shows, during the Q&A. “So the idea that [Arrow and Flash] would fight each other in one of these episodes was one of our earliest ideas. The idea of cast members having that Comic-Con argument on camera — it was one of our earliest ideas.”

The following night, it’s back to Starling City on “Arrow” episode “The Bold and the Brave,” which will feature Team Arrow getting some assistance from Barry and S.T.A.R. Labs’ Caitlin (Danielle Panabaker) and Cisco (Carlos Valdes) as Captain Boomerang (Nick Tarabay) continues to wreak havoc and puts one of their own in danger.

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Below are five things to know about the big “Arrow”-“Flash” crossover event:

1. The “Doctor Who” influence.

Kreisberg and Greg Berlanti, also an executive producer on both shows, took their crossover cues from seminal, classic sci-fi shows of years past. For Berlanti, it was “The Six Million Dollar Man” and “Bionic Woman.”

For Kreisberg, it was the season 4 finale of “Doctor Who,” titled “Journey’s End,” which featured appearances from spinoff shows “Torchwood” and “The Sarah Jane Adventures.”

“In that episode, you saw everybody in the TARDIS,” he said during the Q&A after the screening. “That’s how I imagined seeing everybody in the cortex, or in the foundry. Just to see everybody there together is really amazing.”

2. There won’t be any more crossover events for a while.

Though the “Arrow” and “Flash” universes are more linked than ever, the producers admitted that this will be the last two-parter crossover event for the rest of the season. But, there’s a good reason.

“The finales that we’re building towards on both these shows are so massive, it would just be impossible,” Berlanti explained and teased in one breath.

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Don’t expect other kinds of crossovers anytime soon either. Other big-name DC superheroes won’t be visiting Starling or Central City anytime soon, despite onscreen teases about Bludhaven, a fictional city closely associated with Batman sidekick Nightwing.

“There are things we can do, and there are things we can’t,” said Kreisberg. “I’m a huge fan of Nightwing. But there are cities we can use, and then there’s everything else. You won’t be hearing Gotham or Metropolis on the show anytime soon.”

3. More than one secret will be revealed.

As producers have been teasing, a huge secret about Oliver Queen is revealed during the “Flash” portion of the crossover. But, the final minute or so of the episode will also introduce the next phase of “The Flash” in a big way.

4. The fight scenes were something new.

Needless to say, the fight scenes featuring Amell’s Arrow and Gustin’s Flash are some of the biggest highlights of the crossover event. And just as fighting someone with superpowers took Oliver Queen some getting used to, Amell found himself having to adjust to doing stunts that involved special effects.

“The actual fight with Grant, which we shot over three nights, that was a different experience,” he said.

How much of the time was he fighting opposite Gustin, and how much was solo stunt work?

“I fought the air a little bit,” he joked. “But I kicked the air’s ass.”

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5. A surprise winner in the “Who would win in a fight?” debate.

Perhaps to be diplomatic or avoid all-out war in the divided audience, Grant and Amell both went with a surprising answer to the age-old question of which superhero would win in a fight: Diggle. The episodes pose the question to the character themselves as well, in the aforementioned fight scenes and chatter amongst the other characters, but an answer is harder to come by.

The “Arrow”-“Flash” crossover event kicks off with “The Flash” on Tuesday, Dec 2 at 8 p.m. ET on The CW, and concludes on “Arrow” on Wednesday, Dec 3 at 8 p.m. ET.