When David Thewlis worked on his role as a concentration camp commandant in The Boy In The Striped Pyjamas, he was given a letter written by Rudolf Hoess to his children, shortly before his execution for Nazi atrocities.
Hoess, the SS commandant at Auschwitz, is considered history's greatest mass murderer. He was responsible for exterminating as many as 2.5million people in World War II.
The letter lay about on the kitchen table at the Hollywood home Thewlis shares with actress Anna Friel and their three-year-old daughter Gracie.
'I had some neighbours over and they started reading it,' says Thewlis.
'When they'd finished it, they turned to me and said: "Oh what a beautiful, heart-rending letter this man has written to his children. Who was he? Was he dying? Was he sick?" To which I replied: "Yeah, he was very sick."
But the letter is clearly written by a man with an intense love for his children; it's very articulate, very touching, almost poetic.
'Try and understand a human being - a sensitive human being - who's capable of this! No way can I find it in myself to justify or forgive, obviously.
But my job was somehow to find the humanity in him, and not to see all these people - as the cliche goes - just as monsters. In my research, I came to learn that my character was very much based on fact.'
The Boy In The Striped Pyjamas, a haunting drama adapted from John Boyne's awardwinning novel, centres on the camp commandant's eight-year-old son Bruno, played by Asa Butterfield, who befriends a small Jewish boy in striped pyjamas (Jack Scanlon) on the other side of the camp fence, with tragic consequences.
It's a very different part for Thewlis, who is known mostly for his continuing role as Professor Lupin, the werewolf in the Harry Potter films.
As the Nazi officer whose promotion takes his family from their comfortable house in Berlin to the camp which his young son believes is a sort of farm, Thewlis's character manages to convey both humanity and monstrosity.
He confesses that he was deeply affected by the character, but is also hugely proud, comparing it to his performance as the tortured anti-hero Johnny in Mike Leigh's Naked, for which he won Best Actor at Cannes.
'When you do something well, this is the best job in the world,' he says. 'I've never met anyone who at all resembles the character I'm playing because it's quite unimaginable to understand how one could be a loving father and at the same time leave your children at breakfast, go next door - literally - and spend your time amidst these terrible, terrible atrocities. How do you get your mind around that?'
And Thewlis, 45, is an enormously loving father.
'You can take everything away. You can take the whole acting crew away, but you can't take Gracie away. Nothing else matters but her,' he says, his intriguingly incongruous features (oversized nose, undersized eyes) softening into a wide smile as he speaks about his daughter who accompanied him - 'without crying once,' he tells me - on the 14-hour flight from Los Angeles.
We meet soon after he's finished filming The Boy In The Striped Pyjamas and just as his first novel, The Late Hector Kipling, a hilarious satire on the contemporary art world, is published in paperback.
Thewlis, brought up in Blackpool which features heavily in his book, wrote The End on the final page of the first draft seven years ago, went out to celebrate with a mate and met Anna Friel. They've been together ever since.
'The attraction was something simple really in that we were from the same part of the world, from Lancashire,' he says.
'There was something very familiar about her. She was like the girls I'd grown up with. Before Anna, I'd had a few relationships and I'm glad I've been around a bit. I know where it's gone wrong or know who are the wrong people for me and who I might be wrong for.
'I'd had a relationship with a French girl, a Japanese girl, an American girl, a Filippina and she was there all the time - a Lancashire girl. I thought: "It's a Lancashire girl I was looking for. Why didn't I realise it?"
'Before that night, Anna and I had actually met on an aeroplane going to Cannes. Channel 4 was flying out what they said were 20 of the most prominent actors of the time - it was about 1995 - to promote the British film industry.
'Everyone was there - Jude Law, Kate Winslet, Rachel Weisz. It was weird because some people you knew and some you didn't. I sat there thinking: "We're all very famous on this plane. If this goes down there will be an awful lot of happy equity members."'
And this is the thing about Thewlis: he is very famous, so is his partner, but you can't help but feel he treats it all as one big hoot.
We meet in a rather showy West London hotel into which Thewlis doesn't really fit with his scruffed-up hair and casual charm. He's just flown in from Los Angeles, where Anna is working on the television series Pushing Daisies. The series is a phenomenal success.
Friel, who this year was nominated for a Golden Globe, is a major name in the U.S.
The couple bought a house in the Hollywood Hills next to the Hollywood sign, but hang out more with writers rather than agents or stars. Thewlis says he likes to laugh, and loves funny people.
I wonder how this refreshingly un-starry star copes with his partner's celebrity. 'It's sometimes annoying when a cameraman tries to put a camera up her skirt,' he says, starting to look very annoyed.
'The only time I've got close to hitting a cameraman is when one tried to put a camera up her skirt when she was pregnant - that's sexual assault.
'I said: "What the f*** are you doing?" I could have understood if she was wearing a mini skirt, but to put a camera below a knee-length skirt with a pregnant woman . . . That's weird. It's illegal, I'm sure.'
But he says that mostly, for now, life is 'living in the Hollywood Hills, having a glass of wine when the sun goes down and running up and down the hill'.
Apart from the smoking (he tried to give up but failed), they're both health nuts and have been since Anna discovered she was pregnant - or rather Thewlis did.
'We were living in a cottage in Cornwall where Anna was making a film,' he says. 'She was in the bath and was being picked up in the next five minutes so I was reading the test thing. I double took about 20 times and kept checking the packaging. I said:
'According to this you're pregnant.' She said: 'Stop kidding.' Then: 'Oh my God.' But, she had to go to work and I couldn't go with her because I had a meeting. So, within five minutes of finding out this news we were separated. She was taken off in this car and I was left in the cottage saying, 'I can't believe this.' It didn't sink in for ages.'
Thewlis had wanted a child years before meeting Anna, who worried she might not be able to conceive after suffering an ovarian cyst. 'I'd been a stepparent for about two years with a woman who had a child, and I came to realise I adored children and was good with them. So I was very happy when Anna got pregnant,' he says.
He adds: 'Gracie's birth wasn't very beautiful or spiritual. It was a difficult birth - traumatic for everybody. Anna went through a lot of pain. It was long and tiring. She was having a hard time passing in and out of consciousness. It nearly came to an emergency Caesarean, but at the last minute the baby turned.
'When Gracie eventually came out they laid her down, the cord was cut, they wrapped her on a towel and put her on the scales, but it was three minutes before either I or Anna asked: "What sex is it?"
'You always think you'll say: "Is it a boy or a girl?" But I found myself asking: "Is Anna OK? Is Anna OK?" I was really worried about her.'
Thewlis is passionate about Anna and wants more children, but doesn't think they'll marry. 'It's not a big issue for Anna and I. We've talked about it, but it's more important to have a baby. Marriage doesn't make any difference to the commitment.
'I've been married once [to director Sara Sugarman] and that didn't feel very important. The marriage was a little shaky from the start. We got married on a whim.
'We got carried away and thought: "Wouldn't it be good if we shocked everyone?" I wasn't drinking or smoking then. In fact, that was one of the problems. On the wedding day I didn't drink. I wish I had. It was a bit of a nightmare - not one of the happiest days of my life.
'I was so nervous, I thought: "What are we doing?" We never lived together. We had a romantic idea it would be like Woody Allen and Mia Farrow. She had a place. I had a place. It was a strange marriage.'
Gracie was conceived shortly after Thewlis and Anna went through a sticky patch four years ago when hectic filming schedules kept them apart for five months.
Thewlis's roles in Ridley Scott's Kingdom Of Heaven and The New World with Colin Farrell kept him in Morocco, Spain and Virginia, while Anna was filming The Jury and leading a bachelor-girl life in a New York apartment.
'We didn't see each other for a very long time,' he says. 'When you're on different film sets the phone calls get hard because you don't know the people the other person has become so intimate with, because when you're making films you get close to people.
'You keep in touch, but you're not living the same lives, which would put any relationship under strain. When you come back together again for the first few days it seems strange. It's like you look at each other, but you look different. Five months is too long, particularly now we've got a daughter.
'We don't spend five months apart now and it's of worked out. I have turned some things down and Anna the same.
'I am a changed man because of Anna and Gracie. Anna changed me for the better, from a rather wild, restless character into a gentle soul. Gracie has changed me, too. I look at her and am filled with love. I cannot bare to be parted from either of them.
'I suppose you could say Anna and I are yin and yang. She's gregarious, very vivacious, very social, very tactile, very outgoing.'
But he adds: 'She's also a big worrier. She worries about her parents' health. She'll worry that she's not doing a good job. She'll worry that we've just bought a house in Los Angeles but haven't got enough time to spend there.
'She worries about Gracie. She worries about me. She worries she'll get too tired and not be able to do the job. She'll worry if the job is right, isn't right. If it goes into two series she'll worry about being in Los Angeles for two years.
'I don't worry. I'm more stoical. Of course I have insecurities. I fear getting older. I fear death and illness. I'm not prone to depression, but I get depressed because everybody gets depressed. Suddenly I'm away from my family or doing a job I'm not enjoying.'
I wonder how they'll cope when Gracie goes to school. They'd had their eyes on a delightful school near their home in Windsor where the playground runs into the woods, but that was before Anna's huge success with Pushing Daisies. He shrugs. 'I don't know. We're going to have to take it a day at a time and see.
'We do have this worst case scenario: Scorsese offers me a part in a film in Australia for five months and the Coen brothers offer Anna a big part in Mexico for five months - the same five months. What are we going to do? One of us would have to not do the job.'
He shrugs again. 'I like time off. I'm not a workaholic. I'm really happy when she goes to work and I stay at home with Gracie and I write. It's perfect. That Philip Larkin thing about the enemy of the artist is the pram in the hall. I don't remember the exact quote, but years ago I met Billy Bragg and we talked about that quote.
'His view of that was he didn't believe it at all. He's got several children and loves it and finds it inspiring.
'He gave me a poem he wrote that was a pastiche of Larkin's "They f*** you up, your mum and dad". He changed it to "They tuck you up, your mum and dad" and rewrote every line to make it a positive, loving, parental poem. It was a beautiful thing. You can have the time to be an artist and a parent.'
And, again, those intriguingly incongruous features soften into a wide smile.