From the moment he read the script for The Railway Man, Jeremy Irvine knew it was something he had to do.

Based on the memoirs of the late Eric Lomax, the film will chronicle the experiences of the former Edinburgh soldier on the infamous Burma-Siam "death railway" during World War Two, his torture at the hands of the Japanese and his suffering afterwards.

Irvine, who can be seen in Great Expectations, opening this week, will portray the young Lomax, with Colin Firth filling in his later years. But the role left an indelible mark on the young Irvine, who felt a tremendous burden of responsibility to try and do the story justice.

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"I lost a lot of weight for that movie, far more than I should have done," he said. "I've never put myself through that sort of stuff for a film but it felt worthwhile ... I think it's something that needs to be done and needs to be done well.

"All these things like weight loss and stuff like that. It's just the sort of commitment you have to do. There's nothing pretentious about it, it's not about trying to be earnest. It would be insulting not to."

Irvine met Lomax and his family as part of his research for the role and was humbled. He even goes so far as to say "we'll never do that story justice", given the enormity of Lomax's journey.

Lomax was a signals officer who was captured along with many other Allies in Singapore before ending up in the Thai town of Kanchanaburi, where he worked on the railway link to Burma. He was regularly tortured, suffering broken arms and cracked ribs and being water-boarded. But he survived only to suffer more torment upon arriving back home, where he was unable to talk about his experiences. It was only through his eventual relationship with Takashi Nagase, a Japanese interpreter who felt shame for his involvement in the mistreatment of Allied PoWs, that Lomax was able to slowly forgive, if not entirely forget.

Irvine feels a lot of anger for what he describes as a dark chapter in British military history.

"In Australia, for example, there is a specific day to remember the tragedies that happened on the Burma Railway.

"In England, soldiers on their way home had to sign a form saying they wouldn't talk about it and not only that, that they were of A1 health, which meant they could never go to the army for help for their mental and physical ailments afterwards. In a way, it's the most disgusting abuse by the British military of people.

"A man died for every sleeper of railway that was laid between China and Burma [national archive records state 12,600 Allied PoWs and 85,400 civilian slave workers]. It's truly colossal and something people just don't seem to know enough about. I certainly didn't know enough about it."

Irvine met other survivors and shot scenes in the actual cuttings that had been carved out by the men. And the ensuing film, directed by Jonathan Teplitzky and due for release next year, is something Irvine feels proud to have been a part of.

In fact, making films that matter is something Irvine holds dear. Having shot to prominence as the lead in Steven Spielberg's War Horse, he could have opted for lucrative blockbusters. Instead, he took time to pursue a couple of passion projects.

"I didn't work for six months because I wanted to make Now Is Good and Great Expectations as my next movies, which was – looking back on it – incredibly arrogant and probably quite stupid," he laughs.

"But now I'm so glad I did. There was a lot of temptation but I think I realised quite early that being famous wasn't something I wanted to do. I'd never gone into acting to make money."

In Great Expectations Irvine has the central role of Pip, alongside an ensemble of British stalwarts, from Helena Bonham Carter and Ralph Fiennes to Sally Hawkins and Robbie Coltrane.

Irvine insists it is a tale worth re-telling, particularly as there hasn't been a movie adaptation since David Lean's version in 1946.

"We're not making a period movie here, we're making a modern movie in period clothes," he insists.

And he was taken by the script. "It was so real and honest and so different from a lot of the stuff I'd been reading that was coming out of some of the more commercial sides of the industry. It had so much depth to it."

With Great Expectations and The Railway Man behind him, Irvine is now pursuing another passion project, a self-penned documentary on the life of World War One pilot Albert Ball, a VC.

"I'd thought about writing a script but it's almost so unbelievable that to make a [fiction] film, people would just think: 'Oh, you're taking licence with the story.' It's so extraordinary that I want to tell it for real."

It's an amazing turnaround for a man who claims that when he left school he could spell 'fudge' with his GCSE results.

Great Expectations opens in cinemas tomorrow. The Railway Man is out next year.