She is explaining the effects of water-boarding and what it does to a person. She is talking about nightmares and hurt, and the need for revenge.
And she is telling me about the day she thought her husband was going to kill another man. "He would have garrotted him," she says. "My husband had been taught how to kill."
Listening to Patti Lomax explain all this, in careful, horrific little detail, is an extraordinary experience.
She is a small and neat woman and as thin as a pencil, which is probably why it is Nicole Kidman who plays her in the new film The Railway Man.
On the table in front of her is a glass of water but she doesn't drink it because she's concentrating on getting the whole story out: the story of her late husband Eric, how he ended up in a prisoner of war camp, what he saw there, what the Japanese did to him and how he ended up in Thailand some 50 years later plotting to kill one of the men who tortured him.
The film that tells the story features Colin Firth as Eric, but although Patti says she's flattered to have Nicole Kidman playing her, she doesn't need all the paraphernalia and brightness of a movie to tell us what happened.
She can do it all much better by herself, starting with how she met Eric on a train heading for Glasgow in 1980.
"I saw this gentleman standing on the platform at Crewe," she says, "and although it sounds odd, human beings occasionally get an insight, a feeling about things, and I did then.
"It was like an inner knowledge that this person would mean something to me in the future, so it was no surprise when he got into the same carriage."
Eric immediately spotted that Patti was reading a copy of The Observer Book Of Maps. Together with trains, maps was one of his obsessions and it gave them something to talk about.
"He was so interesting," says Patti. "He was telling me about the different towns we passed through - he was obviously a historian and he had a very light sense of humour that was most attractive.
"It wasn't love at first sight though - it was the map book that did it and we laughed about it many times afterwards. It wasn't the beautiful me, it was the fact I was interested in maps."
One of the other reasons Patti and Eric connected was they were both in difficult marriages and feeling rootless.
Eric had grown up an only child in Edinburgh and spent much of his childhood beside railway lines and at stations indulging his interest in trains; for him, the railway network was a mysterious, mechanical paradise.
This love of detail and the precision of railways would also, much later, get him into the most terrible trouble on the worst railway in the world: the Thailand-Burma Railway during the Second World War.
Patti, on the other hand, was English and at the time she met Eric had been living in Canada.
She and her first husband Raymond had emigrated there with their four children in 1968 but by the time the children were teenagers, they found they had little to talk about.
To get a break while her mother-in-law was visiting, she came for a holiday to Scotland, met Eric and found that they had a lot in common.
"We were certainly rootless at the time," says Patti, "because his marriage was bumbling along, staggering along, not through any fault of theirs. Immediately after Eric returned from the war, his fiancee expected the same man that had gone to war to come back. They didn't stand a chance."
After the first meeting on the train, Eric and Patti went out to lunch and the relationship developed; three years later, they were married and Patti came to live with Eric in Edinburgh.
He had told Patti that he had been a prisoner of the Japanese during the war but had said nothing more and the first indication she had of the depth of the wound his experiences had left came after they were married.
"It was on our honeymoon night," she says, "the first time we slept together and he had this massive nightmare.
"That was the first indication I had because over the years he'd learned to cover it up so well.
"It was simple things like going for a new bank account. In those days, you had a table between you and the bank manager and the usual questions, full name, date of birth, gave him a flashback to how it used to be when he was interrogated and he just walked out."
It was all pretty confusing for Patti who was also uncertain how much to push for details and how much to stand back. In the words of Eric himself, in his book The Railway Man which inspired the film, he had what he called sudden triggerings of frightened anger or long, icy silences during which he would shut out everyone, including Patti.
"It was frightening and confusing," says Patti, "and by that time I was really in love with my husband and I wanted to help him because he was in terrible pain, but how do you broach it? It took many years."
Eventually, two things happened which accelerated events: first, an offer of medical help, and second, a sudden message from one of the men who had tortured Eric when he was a prisoner in Thailand.
The offer of help came from the Medical Foundation For The Care Of Victims Of Torture, now known as Freedom From Torture. Eric and Patti had seen a report about the foundation in the papers and got in touch, pushed gently by Patti.
"Eric knew that if he didn't do something to help himself there was a possibility he could lose me," she says. "He wouldn't have done, but he got that feeling - and so he wrote to the foundation and asked if he could come and see them."
For almost two years, Eric and Patti travelled from their home in Berwick-upon-Tweed to London where they attended counselling sessions at the foundation and, slowly, Patti began to learn the details of her husband's experiences during the war.
He had joined the Royal Signals in 1939 and as a young second lieutenant was sent to Singapore, where the British knew an assault by the Japanese was possible, if not probable.
When the attack came, the British surrendered and with thousands of others, Eric was taken prisoner.
It was then that they began to hear rumours of a grandiose scheme which the Japanese were planning - a railway between Burma and Thailand which could be used to transport supplies.
British engineers had looked at the route and rejected it as impossible but the Japanese took a different view and were determined to build it, using their prisoners of war as labour.
Eric was sent north to Ban Pong prison camp in Thailand near the site of the railway, where, secretly, he and a few other of the prisoners used what they could find to build a primitive radio to listen to British broadcasts.
When the guards found the radio, Lomax and four other officers were singled out to be punished. First, they were made to stand out in the sun for hours, then, one by one, they were beaten. Lomax said watching his comrades being beaten - two of them to death - and waiting for his own turn was the worst moment of his life. Of the attack itself, he remembered the boots on the back of his head, the crack of bones snapping and teeth breaking, and the flashes of light that burned and agonised.
And then it got even worse as Eric moved through the Japanese circles of punishment. He was taken to another camp where he was interrogated and tortured by officers who were determined to find out all they could about the radio and a map of the area around the camp which Eric had made.
One of the Japanese officers was a young man called Nagase Takashi. "Lomax," he said, "you will be killed shortly whatever happens. But it will be to your advantage in the time remaining to tell the whole truth."
When Lomax did not tell the truth, he was tortured, using the ancient - and modern - method of water-boarding, used by the CIA on prisoners in Guantanamo Bay, in which water is poured into the mouth and the prisoner is taken to the verge of drowning. From there, he was taken to a court martial and once again sent to prison.
Patti did not know any of these details until the counselling sessions at the Medical Foundation, but Eric revealing them was the first step to the remarkable changes that were coming in his life.
The second step was a sudden, unexpected reconnection with Nagase in 1991, which began with a book given to Eric by a friend. Patti remembers reading the book, Crosses And Tigers, which was written by Nagase and described his experiences during the war.
It included a description of Eric's water torture and Nagase said he could not stop shuddering every time he recalled it. He also said he regretted his part in the torture but towards the end of the book described a moment in a war cemetery when he was encompassed in a golden light and felt forgiven.
It infuriated Patti when she read it.
"I was so angry," she says, "How could he feel forgiven when Eric had not forgiven him? That was how I felt - nasty little man feeling he was forgiven. Oh, I rarely become angry but when I go, look out."
After a few days, Patti asked Eric's permission to write to Nagase and when she did, she told him exactly how she felt and a little while later he wrote back.
"He said that the dagger of my letter had pierced his heart and he sounded so genuine," said Patti, "but Eric still didn't want to write to him."
After a while, though, the two men did start writing to each other although Patti was worried that Eric's thoughts were not of reconciliation but of revenge.
"I began to suspect it wasn't quite all that it seemed on the surface on Eric's part," she says, "that he was playing a game and I was quite right. He agreed to meet Mr Nagase but admitted later his motive was to kill him. He would have garrotted him, he said."
What changed Eric's mind was meeting Nagase in Thailand when he could see that his former Japanese tormentor was genuinely remorseful. Eric did not forgive him immediately though.
"It took some time," says Patti. "He wanted to make sure he really was what he seemed. But it was almost as though Eric reverted back to being an army officer - he sort of took charge of the situation."
Almost 20 years on from that meeting in Thailand, two years on from Eric's death, and just a matter of days before Christmas, when forgiveness is encouraged, Patti admits, strangely, that in the end it was she who found it harder to move on from the thoughts of revenge.
"I had seen the effects of this man's earlier behaviour," she says, "and I really had to sit down and give myself a good talking to, that really I had no right to feel like this if Eric, being the man that he was, had managed to genuinely forgive. Who was I to continue with it?"
However, she still has little patience with those who appear to preach forgiveness automatically. It has to be worked for, she says; it has to be earned.
"But at the same time," she says, "Eric did believe hatred didn't hurt anybody else but the person who was doing the hating.
"That was the conclusion he came to later on."
Patti now hopes the film will pass on this message; she also hopes it might make a contribution to the end of the use of torture around the world.
"Eric was desperately upset that the US government gave permission for water boarding to be used and what the film could do is open up a bit of a debate - it's one thing the public being told that these terrible things go on, but it's seeing what happens that can change things." n
The Railway Man (15 ) is in cinemas from January 10.