Shortly after CBS' Supergirl transitioned to The CW for its second season, rumors started spreading about a crossover with this program and the CW's superhero lineup, Arrow, The Flash and Legends of Tomorrow. While that program has since been confirmed, it was also revealed that there will also be a musical crossover between The Flash and Supergirl. Both of those show's stars, Grant Gustin and Meissa Benoist are accomplished singers who have both appeared on Glee before their superhero shows, and today we have more details about both crossovers.

It was previously revealed that the Flash and Supergirl musical crossover will largely feature cover songs, but the producers are trying to find a way to include some original songs as well. IndieWire caught up with Greg Berlanti, who serves as an executive producer on all four shows, who revealed that this crossover episode has already been written.

RELATED: The Flash Loses Original Cast Members Tom Cavanagh and Carlos Valdes
"We just finished writing the fall crossovers. And now we're trying to figure out how to produce them. That's probably the most challenging thing we do all year. And now we're doing it across three shows! But next week we'll have to start talking about clearing music. I have a few ideas for tone and style in my head but we're just starting to talk about what that can be. I do want to try and get an original piece of music written. As we make a deal on that we'll probably make some announcements on the original songs."

When asked if they have been talking with Lin-Manuel Miranda, who created and starred in the Broadway sensation Hamilton, Greg Berlanti wouldn't confirm, but he did say that he has spoken with someone they are "really excited about." He also confirmed that the musical crossover will span two episodes, with Supergirl airing on Monday nights at 8 PM ET and The Flash airing on Tuesday nights at 8 PM ET. When asked about the four-show crossover, Greg Berlanti confirmed that Supergirl will be featured, but it will largely focus on Arrow, The Flash and Legends of Tomorrow.

"You really are trying to run a single production across three different productions. But they're run as three separate entities. We have to figure out when we're borrowing one actor from where. We're telling one cohesive story; Supergirl will participate, but the storyline doesn't actively begin there. There are some characters who show up in her episode, but the story begins with The Flash episode and goes to Arrow and Legends of Tomorrow. We're just getting into designing the bad guy for it, and we start now but it doesn't air until the end of November. We will put a lot of time between now and then figuring out visual effects sequences. Just today I was holding the three scripts back-to-back - that's 180 pages of material. It's a three-hour story, almost a miniseries."

There will be a few characters like Katie Cassidy's Black Canary and Wentworth Miller's Captain Cold who have signed deals with The CW that makes them series regulars on all four shows, so they will certainly figure in to the crossovers. The Flash will kick off The CW superhero lineup on Tuesday, October 4, followed by {Arrow{ on Wednesday, October 5, {31} on Monday, October 10, and DC's Legends of Tomorrow on Thursday, October 13. It hasn't been confirmed when these crossover shows will air, but hopefully we'll find out very soon.