There has been a lot of talk about the upcoming musical crossover episode of the Flash. Part of the discussion has been the mini-Glee reunion of Grant Gustin, Melissa Benoist and Darren Criss… but that's not all the connections as we learn through Tiffany Smith from DC All-Access. Criss and Carlos Valdez went to college together and Victor Garber and Jesse L. Martin have been friends for twenty years… and though the finally got to work together after all that time, this will be their first chance to sing together. Criss' Music Meister character brings together Barry and Kara into a fictional world where people sing and dance… and if they die in this world they die in real life.
You'd think doing an episode like this would be a one-off and outside of the regular story, but the writers of these series have found ways to move the main stories along even when doing crossovers.