The characters in the DC shows on The CW, The Flash and Arrow, co-exist in the same universe, allowing for plenty of crossover opportunities. The CW has an annual Arrowverse crossover special that began in 2014. The crossover special has since extended to include Legends of Tomorrow and Supergirl. However, these specials are not the only crossovers in the Arrowverse. Throughout each season, characters from The Flash, Arrow, and Legends of Tomorrow guest star in the other shows.
It was recently reported that Katee Sackhoff has been cast in season 4 of The Flash, appearing in the fifth episode, and that the episode would feature Felicity Smoak (Emily Bett Rickards) to serve as a mini-crossover with Arrow. Executive producer Todd Helbing has now revealed more details to TVLine on what to expect from the episode:
It’s really fun. The girls [including a visiting Felicity Smoak] go and have a night out in Central City alone, without the guys. And they get into some trouble.
Helbing did not reveal if there was a particular reason for the "girls night out," but some fans are already suspecting that it could allude to a bachelorette party for Iris West (Candice Patton). As for which female characters will be seen with Felicity, Iris, Caitlin (Danielle Panabaker), and Tracy Brand (Anne Dudek) are all likely choices. We can guess that the "trouble" encountered by Felicity and the others is Katee Sackhoff's character, Blacksmith. This episode would be something different for the series, since The Flash has not had many opportunities to put focus on the relationships between these characters. The promise for a "fun" episode is also in line with Grant Gustin's remarks about the tone of season 4.
Of all the supporting characters in the Arrowverse, Felicity has been involved in the most crossovers. Aside from participating in the annual crossovers along with most of the cast of Arrow, Felicity was also used as a potential love interest for Barry in the first season of The Flash. She visited Central City again toward the end of season 1 with Ray Palmer (Brandon Routh) to ask for help with his Atom suit. Felicity returned at the beginning of season 3 to talk to Barry about the "Flashpoint" timeline. Plus, she appeared as a vigilante in season 2 of Legends of Tomorrow when reality was rewritten by the Legion of Doom.
Season 4 of The Flash premieres on October 10 on The CW. Season 6 of Arrow premieres October 12.