The 46-year-old actor plays down-and-out former Secret Service agent Lucas Reyes in director James Madigan's film and was keen to push himself to the limits to see if he was still capable of such physical work in his late 40s.
"(Our producers) also did John Wick and they were like, 'Keanu (Reeves) does his own stunts," Hartnett told US Weekly magazine.
"And I was like, 'Yeah, I know.' And they were like, 'Well, do you wanna do your own stunts? It would really make the movie better.' And I was like, 'Yeah, of course I do.'
"In your forties like I was, not many people are asking you to go out and do your own stunts and film.
"So it was such a unique challenge and also just a fun opportunity to have some of the greatest, most fantastic stunt teams around me helping choreograph ...
"I was like, 'Can I still do this? I wanna see if I can.'
"I wanted to prove something to myself, physically, but also just give somebody pure popcorn entertainment in the best way possible, which is with a lot of humour and a lot of physicality and over-the-top, unexpected surprises. I'm very proud of it."
But there was one stunt Hartnett wasn't allowed to do himself.
"I had a wonderful stunt double who helped preface all of the fights,a" he said.
"And he did one stunt for me that I wasn't allowed to do. So aside from that one stunt, I did everything else.
"That was the one where the character's thrown into the bar. Because the glass was not breakaway enough for production and they were worried that I was gonna get cut badly and then we'd have to stop production.
"So that was the only thing that I wasn't allowed to do. But the rest of it's me. And it was pretty demanding."
Despite the physical nature of the role, Hartnett would happily return for a sequel.
"Listen, if a ton of people see it and a ton of people want us to do a sequel, I would be totally game I really enjoyed it, but at the same time, it was more that we wanted to leave the whole thing open-ended and fun and just have the question marks out there," he said.
"This is a fun character that could be explored further with (my co-star) Charithra Chandran. I think she's so talented and so much fun to work with."
The 40 Days and 40 Nights actor revealed he also has a romantic comedy in the works.
Asked if he'd ever return to rom-coms, he said: "I've got one that I'm working on. I can't say anything about it because it's not happening yet, but I've got a script we've been quietly putting together that would be kind of in the rom-com genre."
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