Colin Firth and Rachel Weisz on the harrowing true story behind new film The Mercy

It was an admirable, if ill-advised dream. Crowhurst invested so much money into the construction of his boat, the Teignmouth Electron, that his wife Clare and four children were left on the brink of ruin, entirely dependent on publicity and the slim hope that he would emerge from the race victorious and claim the £5,000 cash prize.

The Electron, however, was not equipped for the currents of the open sea, and, a few weeks into his journey, Crowhurst realised that he would not be able to continue, leading him down a path of desperation and deceit.

So goes the story of The Mercy, the new film starring Colin Firth and Rachel Weisz – an unglamorous (apart from the indisguisable sheen that the stars themselves give off) and compassionate biopic about a man who finds himself in deep water.

"I felt enormous empathy and admiration for him," Firth tells RadioTimes.com. "I think it's easy for people to have less invested in their view when it’s several decades later. But nevertheless, I didn’t have to make an effort – I just thought the guy was remarkable."

Weisz, who stars as Crowhurst's resilient spouse, adds that society's attitude towards the story has changed in the 50 years since: "I think at the time, from what I understand, the press at least were not kind to Donald," she says.

"But now, I think there’s something in our culture: times have changed where, when I heard about the story, my reaction – and I feel like most people’s reactions – was: my god, he’s extraordinary. That he dared to do such a thing, he got so far. There’s much more empathy, I feel.”

The Mercy, which is helmed by The Theory of Everything director James Marsh, does its best to lay bare the nuances of the story: additional characters serve as nightmarish caricatures of the pressures that prevent Crowhurst from turning around: financial ruin (his sponsor Stanley Best, played by Ken Stott) and public shame (David Thewlis' old-school press agent Rodney Hallworth).

While neither actor was aware of the Crowhurst tragedy before they came across the project in its nascent stage, there were plenty of research materials available to Firth and Weisz, from BBC archive tapes – the broadcasting company had commissioned the sailor to document parts of his journey – to his personal notes on board and his log. Together they presented the duelling narrative that drives the story: the inner turmoil plaguing the sailor and his attempts to deceive the race's adjudicators.

Firth, whose only previous experience as a sailor came when he was eight years old, then tasked himself with learning how to look comfortable at the helm of a boat. “I went out and got lessons on one of these little two-man boats. And I did go out on my own in one of those things in really easy and clement conditions".

This experience contributed to his admiration for the man who inspired the film: "the only thing I don't have in common with Donald Crowhurst are his talents and his skills," he says.

The Mercy is released in UK cinemas on Friday 9th February.