"There's a tenacity you get as an asthmatic where, occasionally in your life, the most important thing in the world is being able to take a breath again," he says, perched on a casual chair at a hotel.
"That puts everything in perspective. Ultimately, when your next breath is more important that anything you can do, there's an element of that that builds your character and makes you stronger."
When the new episodes of Spartacus return on Sunday, McIntyre will be needing all the strength he can muster.
He'll be filling the sandals of the late Andy Whitfield, who played the role until he was felled by non-Hodgkin's lymphoma in September.
"It's bittersweet because you kind of wish it was never there in the first place," McIntyre says.
"You wish the guy you watched in season 1 was still there. But given that's not possible, you have this great responsibility to do this dream job you can't imagine," he says.
McIntyre was a reluctant actor in the first place.
"I got tricked into acting. I studied business in Ireland and I was just trying to find things to do to make friends. It was a world away for me.
"And I had no interest in acting ... This guy told me there was this role ... and I grudgingly agreed to do this thing I didn't want to do at all," he recalls.
"I see my little first opening gambit [in the script]. 'OK, maybe I can handle it.' The next thing, my character comes back dressed as someone else ... I went to the director and I said, 'Who plays my character when he comes back?'
"He says, 'You, obviously.' I said 'Wait, that's not what we signed up for. I'm not an actor. I can't do this.' I've never complained so much about getting a lead," he laughs.
"I hated so much of that, up to the fourth night of performance when on stage sometimes it all clicks, and it's just this experience like no other - where all the players are in the same space and they're all connected in some way, and you get offstage with this buzz.
"And I remember one of my co-stars said, 'Did you just get the acting bug?' I said, 'Don't be silly.'
"I remember calling up Dad and apologising and saying, 'Dad, I think I want to be an actor.' It's also so sweet to flash forward nine, 10 years later and to ring up my father and say, 'Your son's done all right'."
Still, he waited a long time for things to go right.
He graduated with a degree in business and excelled at it.
"I ended up in a job I really liked. The job was a cinema chain, I was deciding what movies went on, so by day I was this struggling actor and by night I was telling distributors, 'I don't know if we can fit that in'."
Before he was hired he assured his boss that acting would not interfere.
"Three months later these guys call up and said, 'We want to test you for the show [Spartacus].' I had to go in to my boss and say, 'You know that thing I said would never happen? It just happened.'
"God bless him he was so good about it. He's still the best boss."
McIntyre's parents divorced when he was 1 and he was reared by his mother and stepfather, who died when Liam was 12.
"My stepdad was an incredible man," he sighs.
His death taught McIntyre (29) a valuable lesson.
"I learned to be a leader. I got to be the man of the house. I really did. I said, 'Mum, hold on, we can get through this together.' There's a lot of that I get to use [as Spartacus].
Spartacus now is pulling disparate members together for this bigger cause - and there's a lot of strange similarities in that experience."
McIntyre had lost 20kg for another role when he was called to audition for Spartacus.
"I was up against these two other massive, hulking men and ultimately, like I was trained to do: just do the work. Just be honest to your character and do the right thing. I was told ultimately that's what people kept coming back to."
Then the hard work of shaping McIntyre into a sword-swinging icon began. He toiled at the gym four hours a day, followed a restrictive diet and took instructions from a gruelling ex-military trainer.
"The head stunt guy of Spartacus was like, 'See where he breaks' essentially, and they put me through things you can't imagine," he says, laughing.
Spartacus: Vengeance premieres on Sunday at 9.30pm on Box.
