TORONTO - Nathan Fillion is at the downtown CTV headquarters, talking about his new series “The Rookie,” when he stops mid-sentence to react to police sirens wailing outside.

“Wait - they might need me,” quips the former “Castle” and “Firefly” star, who hails from Edmonton.

“It's natural now. That's the call.”

Playing a rookie cop and co-executive producing on the new ABC drama series, which airs Tuesdays on CTV, has made the 47-year-old more aware of police activity around him, he says.

He also got in tip-top shape for the role of John Nolan, a divorcee who leaves his construction job to work for the Los Angeles police force after a life-altering experience.

Fillion says he did simple things, like cutting out sugar and carbohydrates, drinking lots of water, and walking at least 12,000 steps a day.

“I have a little watch that would count my steps,” Fillion says. “The only problem is, the numbers are so small and my eyes are bad now, so I couldn't quite read it. I had to have someone else read my watch to tell me how many steps I got in.”

Such is the other realization the role has brought Fillion: his age.

Much of “The Rookie” centres around the fact that John is older than his colleagues. They make fun of him for it, and his commander accuses him of being “a walking mid-life crisis.”

Co-stars include Calgary-born actress and singer Melissa O'Neil, who won the third season of “Canadian Idol” in 2005. She plays John's love interest.

The story speaks to a “new cultural norm we have of people starting over,” Fillion says.

“It's been described as a mid-life crisis and I think it's inaccurate. There's no crisis here. It's that people are experiencing this,” he says.

“Children grow up, so you're no longer required as a parent; people get divorced, so they're no longer a husband or wife; their jobs don't stretch on for 40 years anymore. Things move much quicker now, so change happens and you have to change along with it.

“And this mid-life change is the new norm.”

Fillion himself is noticing some of those changes, which he describes with signature wit.

“I call it 'the daily betrayal' - I always expect the 32-year-old me to look back in the mirror, and that's not happening,” he said.

“I'm now at a point where I can't say so much that, 'Oh, I'm just in my 40s.' I'm pushing 50 now, I'm getting up there.... For most people, that's past the halfway mark. I'm on the downslide now.”

Not that he shows it.

On “The Rookie,” Fillion does plenty of stunts and confirms he is indeed the one running in the police-chase scenes. And he showed his physical might in the recently released short “fan film” “Uncharted,” an adaptation of the popular PlayStation video game.

But his knees aren't happy, he admits.

“These babies are saying 50-plus. They're saying 'Slow down,”' he says. “I remember I was 32 years old, I did something to hurt this (right) knee and I said, 'Oh, that'll be fine in two weeks. It'll heal.' That doesn't happen anymore.

“I do plenty of stunts - I'm just not dying to do them; I die when I do them.”

Fillion lives in L.A. but visits Edmonton often.

“I adore my family, we're very tight,” he says, noting they all recently travelled parts of Europe together.

“My parents still live in the house I grew up in, but my room is my dad's office.... There are all these pictures up, like a shrine. All that's missing is candles.”

He says he wasn't sure he was ready to return to primetime TV so soon after “Castle” ended its eight-season run in May 2016.

But he couldn't turn down a chance to work with executive producer Mark Gordon of “Grey's Anatomy,” and creator Alexi Hawley, who was also a writer and producer on “Castle.”

He was also enticed by the story of a character who wants to do something that affects the world - who wants to matter.

“I can respect that, I can get behind that,” says Fillion. “I can't imagine anything worse than not being relevant to the world around me, and I think that's something we can all relate to.”