Grant Gustin as The Flash
Grant Gustin as The Flash in The CW show TheFlash/Facebook

The Flash mid-season finale will air on 19 December on The CW, after the Arrow/Flash crossover episode.

As previously reported, episode 9 titled The Man In The Yellow Suit, will finally reveal the identity of Reverse Flash.

There are several theories on who the Reverse Flash is in the show. Fans have speculated that either Eddie Thawne (Rick Cosnett) or Harrison Wells (Tom Cavanagh) could be Barry's mother's killer, but Grant Gustin dismissed all the theories.

The Flash star Grant Gustin has hinted at a big upcoming twist related to the villainous Reverse Flash.

Gustin told Entertainment Weekly: "I've heard so many theories surrounding Reverse Flash that are all interesting. I was like, 'Oh, I know what's going on,' but literally no one knows what's going on. Anyone who thinks they know what's going on is wrong. I'm in the in and I thought I knew, but Andrew told me and I was like, 'Holy crap.'"

Gustin also teases about the The Flash and Reverse Flash face off.

"Barry will actually get to come face to face with him, not just running with him, but standing face to face with him, looking at him," Gustin revealed.

"He's vibrating, so he doesn't know who he is. We don't see his face. No characters see his face and the audience doesn't necessarily see his face either, but Barry gets to hear him say that he killed his mom."

The Flash Mid-season Finale Synopsis; Is Harrison Wells The Reverse Flash

Executive producer Andrew Kreisberg also teased a few details about the upcoming confrontation.

"We're building towards a confrontation between Barry and the Reverse Flash."

"I like to think that we've done a good job on both shows of meeting fans' expectations and subverting them," Kreisberg said.

"The show has to appeal to everybody. It can't just appeal to comic fans and it can't ignore them either. We're always treading this fine line of trying to bring something new, honoring what's always been done and turning it into this brand new thing."

Suffice it to say, confirming the horrible truth that this villain did, in fact, kill his mother will cause Barry to lose "his mind a little bit," Gustin added.

"Following that, we have a huge fight that we shot in the football stadium in Vancouver. That was huge."

Also, an extended preview of Flash vs Arrow aired on this week's Tuesday episode. The promo shows Oliver and Barry fight to see who will win.

The Green Arrow vs The Fastest Man alive - Who do you think will win?

Watch the trailer here: