Viewers of The CW’s “Supernatural” will be introduced to a new cast of characters and new setting in the April 29 episode of the monster-hunting series. And if the network likes what it sees, these characters (including rival mob families made up of monsters) and the setting (Chicago) will return for a spinoff called “Supernatural: Bloodlines.”

The episode — known in Hollywood as a backdoor pilot — has been filming in Vancouver. That’s where “Supernatural” films. But it will shoot exterior scenes in Chicago Tuesday through Thursday, according to “Bloodlines” writer and executive producer Andrew Dabb. It has yet to be decided if “Bloodlines” would split time between the two cities (similar to what Showtime’s “Shameless” does) should it get picked up.

“We would love to shoot everything in Chicago,” Dabb said over the phone Monday, “but that remains to be seen at this point. It’s a question of economics.”

What made the producers want to set the “Bloodlines” story in Chicago?

“We started to talk about doing an organized crime angle, and that put us in two places: New York and Chicago,” Dabb said. “One of the producers, Robert Singer, has filmed in Chicago and had nothing but good things to say about Chicago. It just made sense story-wise.”

Dabb said “Supernatural” protagonists Sam and Dean Winchester (played by Jared Padalecki and Jensen Ackles, respectively) will appear briefly in the episode, with most of the focus on “Bloodlines” protagonists Ennis Ross (played by Lucien Laviscount) and David Lassiter (Nathaniel Buzolic). Ross, the son of a cop, becomes a monster hunter like the Winchester brothers after his fiancee gets killed. Lassiter is a shape-shifter who gave up the monster life until the death of his brother pulls him back in.

“We want to take a soap-ier approach,” Dabb said. “It wouldn’t be like ‘Supernatural,’ where there is a different case every week. It would be more serialized with ongoing stories.”

Padalecki and Ackles will be part of the Chicago shoot, but Dabb said they wouldn’t be regulars on the spinoff. It’s more likely that the secondary characters on “Supernatural” would make an appearance than the Winchester brothers due to the actors’ already busy “Supernatural” schedules. Still, there is always the potential for a crossover episode.

“We’ve certainly talked internally about a crossover episode,” Dabb said. “We’d love to do it, potentially in the first season. But it will come down to economics and scheduling.”

“Bloodlines” would begin filming its first season in July if it gets the green light.

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