"It was the first lead role I've done and I didn't take stock of how much it becomes your entire life.
"My typical day was an hour in the gym every morning, going to set, for lunch I'd go to the gym again then when we wrapped I'd go back - six days a week for four months. It was a very unnatural way of being and it was emotionally draining because it was physically draining."
Then there was the challenge of having female co-stars like Browning making the most of the role reversal - being on a set where the onus was on the males to look a certain way more than the women - by scoffing down burgers and steaks while Harington followed a diet he calls "a way of starvation".
It was all a necessary undertaking to perfect his gladiator physique for the film, directed by Paul W.S. Anderson (Resident Evil) following slave-turned-gladiator Milo as he fights to save his love amid the volcanic eruption that destroyed the ancient Roman city of Pompeii.
Harington jokes he drew parallels with Hollywood to get into the mindset of Milo.
"He spent his life in these London pits fighting, surviving and becoming a minor celebrity then gets picked up like talent is picked up to be taken to Hollywood - the Pompeii of [acting]."
After getting his start in London theatre production War Horse, Harington's transition to Hollywood has been a roller coaster since Game of Thrones exploded, becoming a global phenomenon. Its season four premiere is so highly anticipated that it will debut on a stadium-sized screen for 7000 fans in New York this week.
"It's very bizarre," Harington says of his Thrones-fuelled fame and success. "You go where it takes you and Thrones has taken me to doing a movie in Hollywood. I'd be just as happy being taken to doing leading roles on the British stage. Maybe not as rich, but just as happy."
The show's cult following means he is regularly approached by fans, something he is happy to embrace.
However he admits some aspects of being thrust into the spotlight remain a struggle, particularly when he goes home to London.
"When you return to normality sometimes it feels like people are treating you differently. I'm away a lot and get more and more distance from my friends. I don't like it when I haven't seen a friend in a while and they act differently.
"So you have to have your good friends who will never treat you differently, always take the piss and keep your feet firmly planted because this can be a flyaway job where people lose perspective on things."
Harington can't reveal details of the fourth season of Thrones but he says it will feature a strong Jon Snow storyline, plenty of action - and improved fighting skills.
"Pompeii taught me a lot about sword-fighting. It sounds wanky, but it taught me it's just a dance two people have with dialogue in it. When I returned to Thrones all the stuntmen were very happy because I could pick things up quicker."
Yet he has no plans to put those skills to use beyond Thrones.
While constantly presented with action scripts, Harington is now determined to change gears.
"I've told my team, 'Don't even think about sending [an action script]. I don't care if it's the next big thing; I'm not interested.'
"I've reached a point where I feel like I've worked, worked and worked and now I can try to start picking what I do."
Pompeii opens on Thursday.