'Chicago Fire': Taylor Kinney and Jesse Spencer face off in firehouse rivalry
It starts off with the simple fact that Severide is the head of the Rescue Squad -- a group of specially-trained firefighters. "Severide... has kind of an elitist point of view," Kinney explains in the clip.
Meanwhile, Casey is the head of Truck 81. "He's a smart, capable guy, whose aim is to move his team in, do his job, and keep his guys safe," says Spencer. "There's two different teams. They haze each other. It's what boys do."
"I would liken it to brothers, the relationship between Casey and Severide. They're both competitive, they're both really good at what they do, so it's just a healthy rivalry," says Kinney. That's how it was, anyway, until it all went wrong -- when Casey and Severide's buddy, Darden, died on the job, the men turned on each other.
"Chicago Fire" airs Wednesday nights at 10 p.m.