WHOM is forgiveness for? Some reviewers of the film The Railway Man have complained that it focuses too little on the actual story of how Eric Lomax, as a young British Army officer, was captured by the Japanese in the fall of Singapore, and tortured horribly as one of thousands of slave-labourers who were sent to build the Burma Railway. But that misses the point spectacularly. It is not a film about the cruelty of war, but about the long, hard process of what it takes to forgive.

This is why the main character, played by Colin Firth, is a man in his 50s, and the wartime scenes are intercut as extended flashbacks. It is why the torture scenes are, in the words of one rehabilitation expert, Dr William Hopkins, "terrifying without being remotely sensationalist".

The film's script by Frank Cottrell Boyce, the man who wrote the opening ceremony for the London Olympics, is far more interesting than a war movie would have been. This is because it depicts Lomax in a haunted present rather than a tortured past.

Anyone who has read Lomax's book, or press articles about him, knows the end of the story. But, despite that, the film creates and maintains a strong sense of suspense. It comes from the simmering emotional volcano of Firth's performance and the unarticulated powerless torment that Nicole Kidman conjures up as Lomax's wife, Patti.

There is a deep ambiguity in our attitude to forgiveness. We speak with awe of figures such as the Enniskillen bomb-victim Gordon Wilson, or the mother of the murdered Liverpool teenager Anthony Walker, who emerge like lonely milestones in our nation's history. We have more fellow-feeling with those like the Revd Julie Nicholson, whose daughter Jenny was killed in the 7/7 London bombings, who say that they cannot forgive.

But what makes the difference? Firth, in an interview, said: "We all know forgiveness is a good idea, but you do the principle a great disservice if you are trite about it. This film is anything but that. It's about what it costs."

What we all appreciate intellectually, or accept theologically, about forgiveness is hard to embrace emotionally. Lomax's conclusion at the end of the film - "Some time the hating has to stop" - is all the more powerful because of the conflicted, and at times disturbing, journey that he undergoes. There is a purgatory on the way.

Something similar was true of Philomena Lee, a woman who is the subject of another recent film, and who met Pope Francis last week, when the eponymous Philomena was screened in the Vatican. It tells the story of how, as an unmarried teenager in Ireland in the 1950s, she was sent to a church home to have her baby and care for him until he was three - when the child was taken from her, and sent abroad to be adopted.

Single mothers were seen, by both Church and many families, as morally and socially unfit to raise a child in Ireland in those days. The film is part of Mrs Lee's campaign to get her government to change the law, in order to open the files on 60,000 other women whose babies were forcibly adopted. She told reporters: "All my life I couldn't tell anyone. We were so browbeaten, it was sucha sin. It was an awful thing to have a baby out of wedlock."

The Pope, she said, "really made me feel so good inside. . . I had such a sense of relief that I had been forgiven." Yet, some years before this, she had announced that she forgave the Church for what had happened with her son. There was liberation in being forgiven, but perhaps greater liberation in forgiving.

Paul Vallely is Visiting Professor in Public Ethics and Media at the University of Chester.