episode "kind of really revisits what makes our show special" and "there are a lot of special things happening in [the] episode". At the end of October 2018, Tony Todd, who voiced Zoom in the second season, revealed the episode would feature "all the [past] speedster villains" returning "wanting a piece of Barry". In mid-November 2018, The CW announced that the episode was titled "What's Past is Prologue", written by Helbing and Lauren Certo, and released the synopsis revealing that Barry and Nora would be travelling back in time to "gather some key necessities" to stop Cicada. The episode's title was taken from a line in William Shakespeares play The Tempest. Two weeks later, Jessica Parker Kennedy revealed that in the episode Nora gets to see a special moment between her mom and dad "that like really breaks her heart". Patton added, "She gets to see Barry and Iris as we've known to love them over the past four seasons," with Parker Kennedy concluding that "the episode really brings the three of them closer." Grant Gustin said the episode "felt like Back to the Future style time travel, seeing ourselves see ourselves in these other moments through a new lens. It was really cool." Casting Main cast members Grant Gustin, Candice Patton, Danielle Panabaker, Carlos Valdes, Hartley Sawyer, Danielle Nicolet, Jessica Parker Kennedy, Chris Klein, Tom Cavanagh, and Jesse L. Martin appear in the episode as Barry Allen / Flash, Iris West-Allen, Caitlin Snow / Killer Frost, Cisco Ramon / Vibe, Ralph Dibny / Elongated Man, Cecile Horton, Nora West-Allen / XS, Orlin Dwyer / Cicada, Sherloque Wells and Joe West, respectively. Gustin also plays Savitar. Cavanagh also plays Harry Wells of Earth-2 and Eobard Thawne / Reverse-Flash. Martin appears in the episode only in archived footage due to a back injury he sustained prior to the start of filming of the season. LaMonica Garrett as Mar Novu / Monitor and John Wesley Shipp as Barry Allen / Flash from the 1990 television series appear in a pre-credits scene. Shipp also briefly appears as Henry Allen. Filming Filming of the episode began on September 26, 2018, and concluded by early October. Gustin thought it was great that they were able to have Cavanagh direct the episode, saying "[Cavanagh] took such good care; he understands the journey... we've found a lot of details that probably wouldn't have been found otherwise." Arrowverse