For Mark Rylance, the opportunity to track down the truth about his late grandfather’s wartime life seemed a way to get closer to the memory of a loved family member. But delving into that past has also accentuated a painful gulf between them.

“I knew him very well, but there was a lot I hadn’t heard about, though I’d suspected some of it,” the Oscar-winning star of Dunkirk and Bridge of Spies told the Observer before the broadcast of a television documentary that will follow his journey of discovery. “I understand his feelings about the Japanese better now, although I can’t agree with them.”

The acclaimed actor and anti-war campaigner agreed to join forces with the team making My Grandparents’ War, a four-part Channel 4 series marking the 80th anniversary of the outbreak of the conflict, because he wanted to learn more about his mother’s father, Osmond Skinner, and to understand his prejudices, as well as the reasons for his later reticence to talk about the second world war. Rylance knew he had been held prisoner in a Japanese camp, but heard little of the atrocities he witnessed or the hardships endured.

Painting a largely bleak picture of Skinner’s prison camp days, the programme-makers unearth one secret of a much happier kind: Rylance learns that his grandfather also loved acting and took part in several amateur productions as a PoW.

“I knew he had been involved in concert parties and had dressed up; but acting in Shakespeare’s Twelfth Night, which I did myself in the West End later, meant there was an odd synchronicity,” said Rylance. “He must have taken it seriously too, because there was a line someone had noted down about ‘Skinner coming to blows’ in one production. It has never quite come to that for me, but these were all revelations.”

Rylance’s grandfather Osmond Skinner, grandmother Hazel, mother Anne and uncle Richard. Photograph: Wild Pictures/Channel 4

Many of the crueller details of those years are revealed for the first time by the documentary because Rylance’s beloved “Os” had drawn a veil over the period to protect himself and his family. “Often you find that generation don’t like to talk about it much,” said Rylance, who has a strong interest in Japanese culture. “It was difficult for us both later when I got very obsessed with the work of the director Akira Kurosawa and the films of the actor Toshiro Mifune. He would become very quiet and look distressed, and then say: ‘I am sorry about this, old chap. I feel an absolute failure because I can’t forgive them.’”

The reluctance of survivors to speak to relatives about wartime experiences is a central theme of the series, which starts on Wednesday. Rylance’s story follows Helena Bonham Carter’s; further episodes feature Carey Mulligan and Kristin Scott Thomas.

Confronted with evidence of the cruelty of his grandfather’s captors, Rylance found he was unable to condemn them as evil. Instead, he questioned the moral cover given by the Geneva Convention and the concept of fairness in warfare. There is no distinction, he will say controversially to camera, between “war criminals and non-war criminals”, adding that: “I think that’s all a con to get young people to sign up, as if it will be a civilised thing they’re going to. I don’t believe it at all.”

Rylance is sorry, he said this weekend, if his comments offend those who watch the documentary: “My apologies go to viewers with survivors of Japanese camps in their family if they are upset by what I say about forgiveness. I don’t want to disrespect them in any way, but we should not think that the enemy are the only brutal ones.”

Rylance with his father David Waters outside HSBC bank in Hong Kong. Photograph: Wild Pictures/Wild Pictures/Channel 4

Skinner, the son of a London grocer, was working for a bank in Hong Kong when war broke out. He had previously served in the Royal Air Force and so, at the age of 42, he signed up to join a volunteer regiment defending the British-controlled territory from the Japanese.

“Out of a sense of duty to the people of Hong Kong, or to Empire, he took up arms,” said Rylance this weekend. “Some of them were elderly. I don’t know what Winston Churchill was thinking, sending all these civilians up against the Japanese forces. That black Christmas morning the Japanese came down the mountain towards the seaside port of Stanley and more than half of the volunteers died.”

The actor was also surprised by the “imperial tone” of government propaganda, urging British subjects to fight with phrases such as “The Empire is Watching You”.

“Japan had been allies in the first world war, after all,” said Rylance, “and my own grandfather had lived there, in Kobe, for six years earlier, sending his family down to Australia while he worked in Hong Kong.”

Skinner was shot that Christmas Day and then spent almost four years as a PoW, suffering torture and the harsh living conditions that scarred him mentally. “He told me he witnessed atrocities that Christmas Day and in prison, yet he felt lucky as he was not sent to the Burma Road because he was older and short. That saved his life,” said his grandson.

I don't think grandfather would agree with everything I say, but I think he would be interested Mark Rylance

The actor’s anti-war position now influences his choice of parts, he said, as does his dislike of violence: “It is not that I think art and entertainment should always be educational, and it is not that I want things to be censored either. It is more that films and plays should be honest about the impact of these actions.”

Holding art to high moral standards can be difficult, Rylance said, but red lines should be drawn by each actor. He saluted the Royal Shakespeare Company for stepping away from the sponsorship of oil companies, something he had protested against, dropping his own associations with the organisation.

“I am very pleased they are not now linking either their excellent work, or the name of Shakespeare, with those companies. I know these corporations say they are changing, but they are not doing enough. And what they are doing every day is really worse than murder. It has gone beyond the point where they should be allowed to lie about it.”

As a boy, Rylance became close to Skinner when staying in his Sussex home, “a kind of paradise with an orchard”, and he now wishes he could talk to him again about the big moral issues. “I don’t think he would agree with everything I say, but I think he would be interested. We would share a faith, I think, that there is goodness in people.

“Os was a very just man, but he had been through something terrible.”