VANCOUVER, BC. The worlds of The CW's "The Flash" and "Arrow" have been tip-toing around each other since last fall.

First Grant Gustin's Barry Allen was introduced in several Season 2 "Arrow" episodes, with Danielle Panabaker's Caitlin and Carlos Valdes' Cisco making their first appearances in the spring.

Then, the "Flash" pilot couldn't be complete without Barry zipping over to Starling City to get Arrow's blessing on his new superhero alter ego. Emily Bett Rickard's Felicity made a visit to Central City later in the fall.

Finally, though, this is the week that the words of "Arrow" and "The Flash" come crashing together with Tuesday's (December 2) "Flash vs. Arrow" and Wednesday's "The Bold and the Brave."

In October, on the Vancouver sets of their respective shows, "Flash" star Gustin and "Arrow" star Stephen Amell reflected on this collision.

"Here's the thing, Starling City and Central City are very different places," Amell told a small group of assembled reporters. "So when I go to Central City, it's very much, 'Alright, I'll tag along. We'll do things your way.' And then the very next night, in our episode, it's 'I'm sorry, we're not in Central City anymore. We're doing things my way. Things are different here.' So it's a really interesting check point for both shows because we're able to teach these big lessons to the characters. So hopefully Oliver comes out of his trip to Central Cit having learned something and Grant comes out of his trip to Starling City having learned something as well."

Gustin agrees, "I was just talking about this yesterday with a friend and just how Oliver and Barry are so different and me and Stephen are just really different and we really get along outside of work. And it's been fun....Barry and Oliver are kind of butting heads but we're seeing them learn how to work together. It's really funny because they're so different so the conversations they have are hilarious."

It sounds as if the butting of heads may not be entirely figurative and both stars were amused by how different action is in their respective worlds.

"Our titles are never just titles," Amell teases of the aforementioned "Flash vs. Arrow." "You can extrapolate and bet based on the title that we fight. And that whole dynamic, which took place over three nights of shooting, was a very new and different experience for me just from a technical standpoint because 'The Flash' has so many effects. We do our flights. We are, on Monday, shooting the biggest fight we've ever done on Arrow and in an incredibly practical way, I'm shooting 100% of it. I have no double and it's on top of Grouse Mountain up here. So that's crazy and that's us. This was them. I actually had to do a move where I'm going to punch and I had to punch in slow motion and literally do slow motion faces and all that stuff. But all the little things, you can tell that the writers have been waiting for the opportunity to mix these words and mix characters for a long time."

That's pretty much how Gustin explains the differences in stunts-style as well.

"They shoot action sequences pretty much like what you see if what you get," Gustin laughs. "And they're really doing everything. And we do a lot of plate shots that are empty shots over the area that we're going to be in and they're putting us in later in post. I do a lot of the fighting that...I don't have to do it full speed and they'll ramp it up. Then a lot of people have to freeze and I keep moving and I have to clear frames, step back into frames. It's really tedious stuff that we have to do. On theirs they learn the fight choreography and they shoot it from the perfect angles and what you see is what you get. When I went to their show they were doing our Flash special effects on Arrow. And we brought our special effects team over so they could explain what we were doing. When they came to our show they brought their special effects team so we could kind of have our style still on their show."

One other thing that Gustin and Amell agree upon: Both stars tease that David Ramsey's role in the crossover generates a lot of humor, which should be good for Diggle fans.

But when it comes to humor and the relatively lighter tone of "Flash," I had to ask Amell if this will be an opportunity for the frequently brooding Oliver Queen to get in a bit more levity?

"No," Amell says without hesitation.

"It's interesting. Oliver, he is opening up," Amell insists. "We saw it in the premiere a little bit, that he was a little bit happier. Obviously we're putting that on hold for the time being, which is a slight disappointment that I didn't get to exercise my cheeks a little bit more before we dropped Canary off of a roof, but it's interesting that Oliver is a little bit the curmudgeon when it comes to Barry. It's because he's overly enthusiastic when it comes to being around me, so my default is to be a little bit more cynical. I think that our characters when we're not together is a little bit more towards the center."

The "Flash" and "Arrow" crossover episodes air on Tuesday and Wednesday nights at 8 p.m. on The CW. Check out Amell's answers from the set in video form above.

And below, as a bit of a bonus, check out Amell discussing modifications to Arrow's costume and tech this season and his own status as "a grouch" when it comes to changes.

A long-time member of the TCA Board and a longer-time blogger of "American Idol," Dan Fienberg writes about TV, except for when he writes about movies or sometimes writes about the Red Sox. But never music. He would sound stupid talking about music.