“We didn’t know where or after we would fit her in, but winter finales are at all times fun for these sorts of [obstacles],” Derek explained to TV Line.
“And we wanted it to be an enormous surprise, but not be the tip of the episode, because sometimes you would like people just going, ‘Oh, God, what else is coming?’
Andrea added: “We love this character. We love our villains, however the character is so fun because she just doesn’t see herself as a villain in any respect.
“So it’s like the best way our guys feel about her versus the best way she sees her place at 51. She feels completely connected to 51, and that is a fun thing to explore, too.”
Chicago Fire season 11 continues Wednesdays on NBC within the USA. A UK release date is yet to be announced.