Both men shovelled clay, pushed wheelbarrows full of mud and carried and stacked bricks for hours in intense heat at one of the 35 brick-making factories on the outskirts of the city where GOAL is delivering aid.
Leinster and Ireland number 8 Heaslip said: "In a situation like that, I don't like standing by and just watching. It's just part of who I am.
"Getting stuck in there gave me a sense of realism which I wouldn't have had unless I got involved."
London Irish lock and captain Casey added: "If you get involved with a charity, you have to understand the kind of work that they do and this trip gave me that opportunity."
Entire families are employed at the kilns, including young children and women who are eight or nine months pregnant. They earn the equivalent of €1 for making 2,000 bricks.
Although they rolled up their sleeves for a number of hours, both players admitted that the efforts that they put in were nothing compared to what the people there have to endure on a day-to-day basis.
"We only scraped the tip of the iceberg, really. Physical labour is physical labour, but it is the conditions at the kilns that make it so, so hard," admitted Heaslip.
"There is no air-con, there are no union breaks, these people slog from dawn to dusk under intense heat and humidity. It was so hot there that I was going through about five t-shirts a day."
"At one point I managed to carry eight bricks on my head, but the women working alongside me were carrying 10, so they were beating me," quipped Casey.
"Apart from the weight, you need to balance them, as well as trying to see where you are going. It is hard work."
The two learned how GOAL is providing families at these 35 kilns with essential water and sanitation services and access to basic healthcare and education services.
"When you see what the families have to endure every day, you really begin to understand how important a good education is," said Heaslip.
"The children at the schools we visited are the first people in their family to learn to read or write.
"That is a tool that their parents would not have had, so straightaway, GOAL is providing them with an advantage in life, giving them an extra chance to escape this kind of poverty."
Apart from the kilns, the players visited some of the city's dumps, where GOAL is providing essential assistance to families forced to scavenge amongst the rubbish for anything they can sell for food.
Casey explained: "The first thing that hits you at the dumps is the smell. It is so overpowering it makes you want to gag.
"You want to put your t-shirt over your mouth and nose but you can't because this is where many people live. It is their home.
"GOAL has also provided toilets and showers at the dumps for many of the residents. Again, having a toilet is something that we take for granted - it's a basic human necessity - but it's new for these children.
"Before GOAL installed the latrines, they were using pools of water alongside the dumps."
"The conditions there were horrendous. It was just awful to see small barefooted children sifting through the rubbish for items that could be recycled," continued Heaslip.
The players also visited a rehabilitation centre for children, where GOAL is supporting the manufacture and supply of orthotics and prosthesis to physically-challenged children in need.
They even managed to impart some rugby skills to a large band of very enthusiastic children along the way.
"I really enjoyed the experience and it has certainly given me a better understanding of the kind of assistance that GOAL provides across Kolkata," said Heaslip.
"I really like the way they operate. They go into an area, see what the people need and make sure that they get it, whether it is water, health services or education.
"It is a very simple, practical and business-like approach and, as I have seen for myself, very, very effective."
Casey added: "If I am lucky enough to have children someday, I will be telling them about what I saw and ensuring that they understand how some people are forced to live.
"I've already been telling the guys back here at London Irish, many of whom would not have had the opportunity to do something like this. They have all been fascinated. The week will stay with me for the rest of my life."
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