Justin Voight first appeared on Chicago Fire, episode 1.03: Professional Courtesy, portrayed by Joe Sinopoli, replaced by Josh Segarra for his run on Chicago PD.
Actor Hanna Dworkin, who plays Gail Corson, previously occurred on Chicago Fire (2012) as parole officer Kendrick, despite the two shows sharing the same fictional universe. She would also return later (season 5) as a DCFS agent.