The upcoming crossover between "The Flash" and "Supergirl" will take place in an episode of "Supergirl" but that doesn't necessarily mean it won't affect the world of "The Flash." According to a report from Cinema Blend, actor Grant Gustin revealed that this crossover will permanently affect Barry Allen aka The Flash for the rest of the season.

"When he comes back, it really ended up affecting my mood," Gustin revealed. "It helped give him [Barry Allen] some confidence knowing he had this other ally out there. He just came back with a more positive outlook than I would've expected him to after breaking the dimensional barrier and disappearing for a full day."

This is in direct response to the events that played out in the past two episodes where Barry managed to close all the breaches leading to Earth-2 but at the cost of losing Jay Garrick (Teddy Sears) to the villain known as Zoom. However, it later turned out that Zoom is a different version of Jay, either from a different Earth or a different timeline.

In the most recent episode, where Barry faced the speedster known as Trajectory (Allison Paige), he discovered that there was a drug called Velocity-9 and that it could make a speedster even faster, although it may also lead to addiction and eventually death, hinting that Zoom is sick and dying and needs his speed to stay alive.

However, crossing over with "Supergirl" will now affect Barry Allen's mood and outlook in life moving forward, hinting that he'll be energized and ready to take on Zoom when he returns to Central City in the upcoming episode of "The Flash."

According to a report from Variety, this will be the first time Grant Gustin and "Supergirl" star Melissa Benoist will actually be working together despite both of them being on "Glee." According to Gustin, they did share at least one frame together but they never managed to have eye contact.