Parsons -- author of best-selling novels including "Man and Boy" -- lived at the airport for seven days last year when he was Heathrow's "writer in residence." "I loved it," says Parsons. "I enjoyed living at the airport, I enjoyed the romance of opening my curtains and there was the runway. It was fun."
During his stay he spent time with pilots, police and security services, but one of the highlights was a visit to the Animal Reception Center (ARC) -- what Parsons calls "the true London Zoo."
"Everything passes through the ARC so you get rattlesnakes that some idiot smuggled back from the Nevada desert, you get polo ponies from Argentina and you get cheetahs that are en route from Ghana to Sweden that are in a box that's too small and seized," he says.
Parsons recalls following an ARC worker on a call out to a plane returning from Las Vegas to capture two smuggled rattlesnakes that had got loose.
"All the mad stuff happens on quite a frequent basis. (The ARC employee) said the best thing to pick up a snake that's loose on a jumbo is a pillowcase and he was just wonderfully matter-of-fact about it," he says.
But Parsons also saw the darker side of the transport hub and says he was shocked to discover the lengths some people go to in an attempt to illegally stay in Britain.
"I saw a man from Iraq, granted asylum in Greece, but he wanted to come to the UK," recalls Parsons. "Under the terms of his residency in Greece he wasn't allowed to come to the UK and he ate his travel documents as he was getting off the flight so he could say, 'No, I've just arrived from my war-torn homeland.' It's quite common.
"The experience of being there was so extreme, to be with an immigration officer when they're interviewing someone and they know they've got 50 condoms full of cocaine inside them and they will not admit it.
"You don't see humanity at its very best when you're at an airport ... You see the immigration and customs officers are lied to most of the time. They see (humanity) probably at its worst, at its most criminal and venal."
Read more: A day in the life of Heathrow
Another surprise for Parsons was seeing a woman arrive on a flight from overseas dressed in a wedding gown because she wanted to marry Prince Harry -- an apparently common occurrence (it used to be William before he got hitched, adds Parsons.)
The incident formed part of one of the short stories in Parsons' book "Departures: Seven stories from Heathrow," inspired by his stay at the airport. Another tale was inspired by his discovery that the airport's air traffic control was manned by "youngsters in their cargo pants."
Parsons says: "I thought that venerable elderly gentlemen that looked something like a BBC presenter would be in charge of the skies and of course, it's all the children of Mark Zuckerberg, kids that have grown up with computers.
"There's that moment where you look around the air traffic control tower, this wonderful panoramic view of the sun coming up, the skylight of London 50 miles away, (and you think) where are all the grown ups?"
But Parsons was impressed with their professionalism and says many of Heathrow's employees were childhood airplane enthusiasts who never lost their passion.
"The thing that is most striking about Heathrow is how totally obsessive the people are who work there," he says. "It's not just another job, for the firemen, for the pilots, for the air traffic controllers, for the people at customs, the people at immigration. They really feel as though they're in touch with something epic, something that's bigger than they are."
And he was impressed by the sheer size of the airport. Parsons says: "The thing about Heathrow is that it's a secret city more than an airport. It's a place where 70,000 people work. It's a place where 75 million people pass through every year.
"It's got its own methodology, its own rituals and is far more than just another hub. It's like another city."